Fix inter-branch-merge to create proper merge commit when using ResetToTargetPaths

.github/workflows/scripts/inter-branch-merge.ps1

@@ -126,10 +126,7 @@ function ResetFilesToTargetBranch($patterns, $targetBranch) {
         return
     }
 
-    # Configure git user for the commit
-    # Use GitHub Actions bot identity
-    Invoke-Block { & git config user.name "github-actions[bot]" }
-    Invoke-Block { & git config user.email "41898282+github-actions[bot]@users.noreply.github.com" }
+    # Note: git user.name/email must already be configured by the caller
 
     # Track which patterns had changes
     $processedPatterns = @()
@@ -220,13 +217,97 @@ try {
     Write-Host $committersList
 
     $mergeBranchName = "merge/$MergeFromBranch-to-$MergeToBranch"
-    Invoke-Block { & git checkout -B $mergeBranchName  }
 
-    # Reset specified files to target branch if ResetToTargetPaths is configured
+    # Track whether we created a merge commit (affects push strategy for PR updates)
+    $createdMergeCommit = $false
+
+    # When ResetToTargetPaths is configured, we attempt to create a proper merge commit
+    # so that the target branch content is included. If there are conflicts outside
+    # the pattern list, we fall back to the original source-only behavior so that
+    # GitHub's merge button surfaces the real conflicts to the reviewer.
     if ($ResetToTargetPaths) {
-        $patterns = $ResetToTargetPaths -split ";"
+        # Configure git user for the merge commit
+        Invoke-Block { & git config user.name "github-actions[bot]" }
+        Invoke-Block { & git config user.email "41898282+github-actions[bot]@users.noreply.github.com" }
+
+        $patterns = ($ResetToTargetPaths -split ";") | % { $_.Trim() } | ? { $_ }
+
+        # Start from the target branch and merge source into it
+        Invoke-Block { & git checkout -B $mergeBranchName "origin/$MergeToBranch" }
+
+        # Try a clean merge first
+        $mergeOutput = & git merge --no-ff "origin/$MergeFromBranch" -m "Merge branch '$MergeFromBranch' into $MergeToBranch" 2>&1
+        $mergeExitCode = $LASTEXITCODE
+
+        # Always log merge output for CI diagnostics
+        if ($mergeOutput) {
+            $mergeOutput | Write-Host
+        }
+
+        if ($mergeExitCode -eq 0) {
+            $createdMergeCommit = $true
+        } else {
+            # Get list of conflicting files
+            [string[]] $conflictFiles = & git diff --name-only --diff-filter=U
+
+            # Abort the conflicted merge before proceeding.
+            # Use plain call (not Invoke-Block) because git merge --abort exits 128
+            # if there is no merge-in-progress (e.g. a non-conflict git failure).
+            & git merge --abort 2>&1 | Write-Host
+
+            if (-not $conflictFiles -or $conflictFiles.Count -eq 0) {
+                # Merge failed but produced no conflicts — unexpected git error.
+                # Fall back to source-only branch and let GitHub handle it.
+                Write-Host -f Yellow "Merge failed with exit code $mergeExitCode but no conflicts were detected."
+                Write-Host -f Yellow "Falling back to source-only branch."
+                Invoke-Block { & git checkout -B $mergeBranchName "origin/$MergeFromBranch" }
+            } else {
+                Write-Host "Merge produced conflicts. Checking if all conflicts are within ResetToTargetPaths..."
+
+                # Check which conflicts fall outside ResetToTargetPaths patterns
+                $outsidePatternConflicts = @()
+                foreach ($file in $conflictFiles) {
+                    $covered = $false
+                    foreach ($pattern in $patterns) {
+                        if ($file -like $pattern) {
+                            $covered = $true
+                            break
+                        }
+                    }
+                    if (-not $covered) {
+                        $outsidePatternConflicts += $file
+                    }
+                }
+
+                if ($outsidePatternConflicts.Count -eq 0) {
+                    # All conflicts are in ResetToTargetPaths files which will be overwritten
+                    # by target branch content anyway, so it's safe to auto-resolve them.
+                    # Use -X ours (favor target) as a safety net: if ResetFilesToTargetBranch
+                    # misses a file due to pathspec differences, the merge commit already
+                    # has the target version rather than silently carrying source content.
+                    Write-Host "All conflicts are within ResetToTargetPaths patterns. Auto-resolving with -X ours (favor target)..."
+                    Invoke-Block { & git merge --no-ff "origin/$MergeFromBranch" -X ours -m "Merge branch '$MergeFromBranch' into $MergeToBranch" }
+                    $createdMergeCommit = $true
+                } else {
+                    # There are conflicts outside ResetToTargetPaths. We must NOT auto-resolve
+                    # these because it would hide real conflicts from the PR reviewer.
+                    # Fall back to the original behavior: create branch from source HEAD
+                    # so GitHub's merge button will surface the conflicts.
+                    Write-Host -f Yellow "Conflicts detected outside ResetToTargetPaths patterns:"
+                    $outsidePatternConflicts | % { Write-Host -f Yellow "  - $_" }
+                    Write-Host -f Yellow "Falling back to source-only branch so GitHub surfaces these conflicts in the PR."
+                    Invoke-Block { & git checkout -B $mergeBranchName "origin/$MergeFromBranch" }
+                }
+            }
+        }
+
         ResetFilesToTargetBranch $patterns $MergeToBranch
     }
+    else {
+        # Without ResetToTargetPaths, the original behavior is fine: create a branch
+        # from the source and let GitHub's merge button do the actual merge.
+        Invoke-Block { & git checkout -B $mergeBranchName }
+    }
 
     $remoteName = 'origin'
     $prOwnerName = $RepoOwner
@@ -277,7 +358,36 @@ try {
 
         try {
             if ($PSCmdlet.ShouldProcess("Update remote branch $mergeBranchName on $remoteName")) {
-                Invoke-Block { & git push $remoteName "${mergeBranchName}:${mergeBranchName}" }
+                # Refresh the remote tracking ref for the merge branch so the
+                # human-commit safety check below uses current remote state.
+                & git fetch $remoteName $mergeBranchName 2>$null
+
+                if ($createdMergeCommit) {
+                    # Merge commits create non-fast-forwardable history on each run,
+                    # so we need --force to update the branch. But first check if the
+                    # remote has human-pushed commits that aren't in our local branch
+                    # to avoid silently overwriting manual conflict resolutions.
+                    [string[]] $extraCommits = & git rev-list "$mergeBranchName..origin/$mergeBranchName" 2>$null
+                    if ($extraCommits -and $extraCommits.Count -gt 0) {
+                        Write-Warning "Remote branch '$mergeBranchName' has $($extraCommits.Count) commit(s) not in the local branch. Skipping force push to avoid overwriting manual changes."
+                        throw "Remote branch has unmerged commits"
+                    }
+                    Invoke-Block { & git push --force $remoteName "${mergeBranchName}:${mergeBranchName}" }
+                } else {
+                    # Try non-force push first. If it fails (e.g. remote diverged from
+                    # a previous merge-commit run), retry with --force after checking
+                    # for human-pushed commits (same guard as the merge-commit path).
+                    & git push $remoteName "${mergeBranchName}:${mergeBranchName}" 2>&1 | Write-Host
+                    if ($LASTEXITCODE -ne 0) {
+                        [string[]] $extraCommits = & git rev-list "$mergeBranchName..origin/$mergeBranchName" 2>$null
+                        if ($extraCommits -and $extraCommits.Count -gt 0) {
+                            Write-Warning "Remote branch '$mergeBranchName' has $($extraCommits.Count) commit(s) not in the local branch. Skipping force push to avoid overwriting manual changes."
+                            throw "Remote branch has unmerged commits"
+                        }
+                        Write-Host "Non-force push failed (likely diverged history). Retrying with --force..."
+                        Invoke-Block { & git push --force $remoteName "${mergeBranchName}:${mergeBranchName}" }
+                    }
+                }
             }
             $prUpdatedSuccess = $true
         }