fix: improve branch reference handling and compare with merged state

kustdiff

@@ -39,6 +39,14 @@ function safe_filename() {
   echo "$1" | sed 's/[^a-zA-Z0-9.]/_/g'
 }
 
+function git_with_error_handling() {
+  if ! "$@"; then
+    echo "Error executing git command: $*"
+    return 1
+  fi
+  return 0
+}
+
 function build_ref {
   local ref="$1"
   local output_dir="$2"
@@ -63,28 +71,65 @@ function main {
   validate_root_dir
   validate_max_depth
 
-  git config --global --add safe.directory "$GITHUB_WORKSPACE"
+  git_with_error_handling git config --global --add safe.directory "$GITHUB_WORKSPACE"
 
   # Save current state to restore later
   local current_branch
-  current_branch=$(git rev-parse --abbrev-ref HEAD)
+  current_branch=$(git rev-parse --abbrev-ref HEAD || echo "detached")
+
+  # Check if the branch exists locally or as a remote reference
+  function resolve_branch_ref() {
+    local branch_name="$1"
+
+    # First check if it's a local branch
+    if git show-ref --verify --quiet "refs/heads/$branch_name"; then
+      echo "$branch_name"
+      return 0
+    fi
+
+    # Next check if it's a remote branch
+    if git show-ref --verify --quiet "refs/remotes/origin/$branch_name"; then
+      echo "origin/$branch_name"
+      return 0
+    fi
+
+    # Finally check if it's a valid commit SHA
+    if git cat-file -e "$branch_name^{commit}" 2>/dev/null; then
+      echo "$branch_name"
+      return 0
+    fi
+
+    # If we get here, we couldn't resolve the reference
+    echo "Error: Could not resolve reference: $branch_name"
+    return 1
+  }
+
+  # Resolve the BASE and HEAD references
+  local base_ref_resolved
+  base_ref_resolved=$(resolve_branch_ref "$INPUT_BASE_REF") || exit 1
+  debug_log "Resolved base reference: $base_ref_resolved (from $INPUT_BASE_REF)"
 
   # Build BASE output
   local safe_base_ref=$(safe_filename "$INPUT_BASE_REF")
   local base_output_dir="$TMP_DIR/base"
-  build_ref "$INPUT_BASE_REF" "$base_output_dir"
+  build_ref "$base_ref_resolved" "$base_output_dir"
+
+  # Resolve HEAD reference
+  local head_ref_resolved
+  head_ref_resolved=$(resolve_branch_ref "$INPUT_HEAD_REF") || exit 1
+  debug_log "Resolved head reference: $head_ref_resolved (from $INPUT_HEAD_REF)"
 
   # Create a temporary merge branch
   local merge_branch="temp-merge-$RANDOM"
-  git checkout -b "$merge_branch" "$INPUT_BASE_REF" --quiet
+  git checkout -b "$merge_branch" "$base_ref_resolved" --quiet
 
-  debug_log "Creating temporary merge of $INPUT_HEAD_REF into $INPUT_BASE_REF"
+  debug_log "Creating temporary merge of $head_ref_to_use into $base_ref_to_use (via $merge_branch)"
 
   # Attempt to merge HEAD into BASE
-  if ! git merge "$INPUT_HEAD_REF" --quiet; then
-    echo "Merge conflict detected. Cannot automatically merge $INPUT_HEAD_REF into $INPUT_BASE_REF."
-    git merge --abort
-    git checkout "$current_branch" --quiet
+  if ! git merge "$head_ref_to_use" --quiet; then
+    echo "Merge conflict detected. Cannot automatically merge $head_ref_to_use into $base_ref_to_use."
+    git merge --abort || true
+    git checkout "$current_branch" --quiet || git checkout "$base_ref_to_use" --quiet || true
     exit 1
   fi
 
@@ -94,17 +139,18 @@ function main {
 
   # Compare outputs
   set +e
-  diff=$(git diff --no-index "$base_output_dir" "$merged_output_dir")
+  diff=$(git diff --no-index "$base_output_dir" "$merged_output_dir" 2>&1)
   local diff_exit_code=$?
 
+  debug_log "Git diff exit code: $diff_exit_code"
   debug_log "Git diff output:"
   debug_log "$diff"
   debug_log "End of git diff output"
   debug_log "------------------------------------"
 
   # Clean up temporary branches
-  git checkout "$current_branch" --quiet
-  git branch -D "$merge_branch" --quiet
+  git checkout "$current_branch" --quiet || git checkout "$base_ref_to_use" --quiet || true
+  git branch -D "$merge_branch" --quiet || true
 
   if [[ $diff_exit_code -eq 0 ]]; then
     output="No differences found in kustomize output after merging $INPUT_HEAD_REF into $INPUT_BASE_REF"