Move git command to git/gitcmd (#35483)

The name cmd is already used in many places and may cause conflicts, so
I chose `gitcmd` instead to minimize potential naming conflicts.

modules/git/gitcmd/utils.go

@@ -0,0 +1,14 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gitcmd
+
+import "fmt"
+
+// ConcatenateError concatenats an error with stderr string
+func ConcatenateError(err error, stderr string) error {
+	if len(stderr) == 0 {
+		return err
+	}
+	return fmt.Errorf("%w - %s", err, stderr)
+}