Fix git diff to use reliable commit references

The previous approach used GitHub event SHAs that may not be available
in the repository. Updated to use:
- origin/main..HEAD for PRs (more reliable)
- Fallback to HEAD~1 for pushes when event.before is not available
- Added git cat-file check to verify commit existence before using it

This should fix the "Could not access" git errors and properly detect
changed directories for test execution.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: williamw

.github/workflows/ci.yml

@@ -54,9 +54,15 @@ jobs:
         id: changed-dirs
         run: |
           if [ "${{ github.event_name }}" = "pull_request" ]; then
-            CHANGED_DIRS=$(git diff --name-only ${{ github.event.pull_request.base.sha }} ${{ github.sha }} | grep -v '^\.github/' | cut -d'/' -f1 | sort -u | tr '\n' ' ')
+            # For PR, use origin/main as base
+            CHANGED_DIRS=$(git diff --name-only origin/main..HEAD | grep -v '^\.github/' | cut -d'/' -f1 | sort -u | tr '\n' ' ')
           else
-            CHANGED_DIRS=$(git diff --name-only ${{ github.event.before }} ${{ github.sha }} | grep -v '^\.github/' | cut -d'/' -f1 | sort -u | tr '\n' ' ')
+            # For push, use the before commit if available, otherwise use HEAD~1
+            if [ -n "${{ github.event.before }}" ] && git cat-file -e "${{ github.event.before }}" 2>/dev/null; then
+              CHANGED_DIRS=$(git diff --name-only ${{ github.event.before }} ${{ github.sha }} | grep -v '^\.github/' | cut -d'/' -f1 | sort -u | tr '\n' ' ')
+            else
+              CHANGED_DIRS=$(git diff --name-only HEAD~1..HEAD | grep -v '^\.github/' | cut -d'/' -f1 | sort -u | tr '\n' ' ')
+            fi
           fi
           echo "Changed directories: $CHANGED_DIRS"
           echo "dirs=$CHANGED_DIRS" >> $GITHUB_OUTPUT