fix: Enhance commit existence checks in GitRepository (#23)

- Added a new method `commitExistsLocally` to verify if a commit exists in the local repository.
- Updated the `IsAncestor` method to check for local commit existence before attempting to fetch, improving reliability in ancestor checks.
- Ensured that the fetch operation is only attempted if necessary, and added error handling for cases where commits are still not found after fetching.

These changes improve the robustness of commit comparisons in the Git repository management.

Signed-off-by: Alessandro De Blasis <alex@deblasis.net>

internal/git/compare.go

@@ -19,8 +19,29 @@ const (
 	CommitsDiverged               CommitComparisonResult = 2
 )
 
+// commitExistsLocally checks if a commit exists in the local repository
+func (r *GitRepository) commitExistsLocally(ctx context.Context, commit string) bool {
+	_, err := r.executor.ExecuteCommand(ctx, "git", "cat-file", "-e", commit)
+	return err == nil
+}
+
 // IsAncestor checks if commitA is an ancestor of commitB
 func (r *GitRepository) IsAncestor(ctx context.Context, commitA, commitB string) (bool, error) {
+	// Check if commits exist locally
+	for _, commit := range []string{commitA, commitB} {
+		if !r.commitExistsLocally(ctx, commit) {
+			// Commit not found locally, try fetching
+			if err := r.Fetch(ctx); err != nil {
+				return false, fmt.Errorf("failed to fetch after missing commit %s: %w", commit, err)
+			}
+			// Verify commit exists after fetch
+			if !r.commitExistsLocally(ctx, commit) {
+				return false, fmt.Errorf("commit %s not found even after fetch", commit)
+			}
+			break // Only need to fetch once
+		}
+	}
+
 	_, err := r.executor.ExecuteCommand(ctx, "git", "-C", r.cfg.GitDir, "merge-base", "--is-ancestor", commitA, commitB)
 	if err != nil {
 		var exitErr *exec.ExitError