fix(client): show git's own error instead of exit status

backend/internal/gitrepo/gitrepo.go

@@ -22,6 +22,10 @@ type Repo struct {
 
 func run(dir string, args ...string) (string, error) {
 	out, err := exec.Command("git", append([]string{"-C", dir}, args...)...).Output()
+	var failed *exec.ExitError
+	if errors.As(err, &failed) && len(failed.Stderr) > 0 {
+		err = errors.New(strings.TrimPrefix(strings.TrimSpace(string(failed.Stderr)), "fatal: "))
+	}
 	return strings.TrimRight(string(out), "\n"), err
 }