Fix shallow clone: deepen history for git merge-base in CI

git fetch --depth=1 alone doesn't provide enough history for
merge-base to find a common ancestor. Add --deepen=200 to expand
the shallow clone so --only-changes can compute the correct diff.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

.github/workflows/test.yml

@@ -137,7 +137,9 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Fetch master for coverage diff
-        run: git fetch origin master:master --depth=1
+        run: |
+          git fetch origin master:master --depth=1
+          git fetch --deepen=200
         continue-on-error: true
 
       - name: Download Coverage Cache