fix: remove --depth=1 from SublibraryCI base branch fetch

The detect-changes job fails with "fatal: origin/master...HEAD: no merge
base" on fork PRs because `git fetch origin master --depth=1` only
retrieves a single commit of master history. This is insufficient for
Git to compute the merge base needed by the three-dot diff.

Remove the --depth=1 flag so the full base branch history is fetched,
matching the fetch-depth: 0 already set on checkout.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ChrisRackauckas

.github/workflows/SublibraryCI.yml

@@ -33,7 +33,7 @@ jobs:
             CHANGED=$(git diff --name-only ${{ github.event.before }} ${{ github.sha }} 2>/dev/null || git diff --name-only HEAD~1 HEAD)
           else
             # For pull requests, compare against the base branch
-            git fetch origin ${{ github.event.pull_request.base.ref }} --depth=1
+            git fetch origin ${{ github.event.pull_request.base.ref }}
             CHANGED=$(git diff --name-only origin/${{ github.event.pull_request.base.ref }}...HEAD)
           fi