fix: Adjust regex for diff stats output

The regex for parsing the output of `git diff --cached --stat` was too strict. It did not account for the pluralization of "insertion" and "deletion". This change makes the regex more flexible to handle both singular and plural forms, ensuring accurate parsing of the diff statistics.
---
{"auto-commit-msg":{"language":"rust","version":"0.4.0-dev","model":"gemini-2.5-flash-lite","response_time":1.62,"execution_time":1.65}}

Author: sestrella

cmd/root.go

@@ -72,6 +72,7 @@ var rootCmd = &cobra.Command{
 		if preCommitDetected {
 			commitSource = os.Getenv("PRE_COMMIT_COMMIT_MSG_SOURCE")
 		}
+
 		if commitSource != "" {
 			log.Printf("Commit source '%s' is not empty, skipping commit message generation\n", commitSource)
 			return nil

internal/git/root.go

@@ -28,7 +28,7 @@ func DiffCachedStats() (*Stats, error) {
 		return nil, err
 	}
 
-	re := regexp.MustCompile(`\s+(\d+)\s+files? changed,\s+(\d+)\s+insertions\(\+\),\s+(\d+)\s+deletions\(\-\)`)
+	re := regexp.MustCompile(`\s+(\d+)\s+files? changed,\s+(\d+)\s+insertions?\(\+\),\s+(\d+)\s+deletions?\(\-\)`)
 	matches := re.FindStringSubmatch(string(output))
 	if len(matches) < 4 {
 		return nil, errors.New("TODO")