Make git clone URL could use current signed-in user (#33091)

close #33086

* Add a special value for "SSH_USER" setting: `(DOER_USERNAME)`
* Improve parseRepositoryURL and add tests (now it doesn't have hard
dependency on some setting values)

Many changes are just adding "ctx" and "doer" argument to functions.

By the way, improve app.example.ini, remove all `%(key)s` syntax, it
only makes messy and no user really cares about it.

Document: https://gitea.com/gitea/docs/pulls/138

services/mirror/mirror_pull.go

@@ -32,7 +32,7 @@ const gitShortEmptySha = "0000000"
 
 // UpdateAddress writes new address to Git repository and database
 func UpdateAddress(ctx context.Context, m *repo_model.Mirror, addr string) error {
-	u, err := giturl.Parse(addr)
+	u, err := giturl.ParseGitURL(addr)
 	if err != nil {
 		return fmt.Errorf("invalid addr: %v", err)
 	}