fix: distinguish relative refs from branch refs in git validation

Note: Relative refs (HEAD~N) only need the commit to exist; branch refs
need merge-base resolution. Also provide minimum fetch-depth in error
message.

Author: loadingalias

action.yaml

@@ -392,18 +392,31 @@ runs:
       run: |
         set -euo pipefail
 
-        # Check for shallow clone (may cause inaccurate change detection)
+        # Check for shallow clone
         if [[ -f .git/shallow ]]; then
           echo "::warning::Shallow clone detected. For accurate change detection, use 'fetch-depth: 0' in actions/checkout."
           echo "shallow=true" >> "$GITHUB_OUTPUT"
         else
           echo "shallow=false" >> "$GITHUB_OUTPUT"
         fi
 
-        # Verify merge-base is resolvable
-        if ! git merge-base HEAD "$BASE_REF" &>/dev/null; then
-          echo "::error::Cannot resolve merge-base between HEAD and $BASE_REF. Ensure 'fetch-depth: 0' in actions/checkout."
-          exit 1
+        # Validate base ref exists and is resolvable
+        # Relative refs (HEAD~N) just need the commit to exist
+        # Branch refs need merge-base to find common ancestor
+        if [[ "$BASE_REF" =~ ^HEAD~[0-9]+$ ]]; then
+          # Relative ref: verify commit exists
+          if ! git rev-parse "$BASE_REF" &>/dev/null; then
+            depth="${BASE_REF#HEAD~}"
+            min_depth=$((depth + 1))
+            echo "::error::Cannot resolve $BASE_REF. Shallow clone lacks history. Use 'fetch-depth: $min_depth' or 'fetch-depth: 0' in actions/checkout."
+            exit 1
+          fi
+        else
+          # Branch ref: verify merge-base is resolvable
+          if ! git merge-base HEAD "$BASE_REF" &>/dev/null; then
+            echo "::error::Cannot resolve merge-base between HEAD and $BASE_REF. Ensure 'fetch-depth: 0' in actions/checkout."
+            exit 1
+          fi
         fi
 
     # Run planner and emit selected output mode