Use the type RefName for all the needed places and fix pull mirror sync bugs (#24634)

This PR replaces all string refName as a type `git.RefName` to make the
code more maintainable.

Fix #15367
Replaces #23070 
It also fixed a bug that tags are not sync because `git remote --prune
origin` will not remove local tags if remote removed.

We in fact should use `git fetch --prune --tags origin` but not `git
remote update origin` to do the sync.

Some answer from ChatGPT as ref.

> If the git fetch --prune --tags command is not working as expected,
there could be a few reasons why. Here are a few things to check:
> 
>Make sure that you have the latest version of Git installed on your
system. You can check the version by running git --version in your
terminal. If you have an outdated version, try updating Git and see if
that resolves the issue.
> 
>Check that your Git repository is properly configured to track the
remote repository's tags. You can check this by running git config
--get-all remote.origin.fetch and verifying that it includes
+refs/tags/*:refs/tags/*. If it does not, you can add it by running git
config --add remote.origin.fetch "+refs/tags/*:refs/tags/*".
> 
>Verify that the tags you are trying to prune actually exist on the
remote repository. You can do this by running git ls-remote --tags
origin to list all the tags on the remote repository.
> 
>Check if any local tags have been created that match the names of tags
on the remote repository. If so, these local tags may be preventing the
git fetch --prune --tags command from working properly. You can delete
local tags using the git tag -d command.

---------

Co-authored-by: delvh <dev.lh@web.de>

modules/git/repo_branch.go

@@ -14,14 +14,6 @@ import (
 // BranchPrefix base dir of the branch information file store on git
 const BranchPrefix = "refs/heads/"
 
-// AGit Flow
-
-// PullRequestPrefix special ref to create a pull request: refs/for/<targe-branch>/<topic-branch>
-// or refs/for/<targe-branch> -o topic='<topic-branch>'
-const PullRequestPrefix = "refs/for/"
-
-// TODO: /refs/for-review for suggest change interface
-
 // IsReferenceExist returns true if given reference exists in the repository.
 func IsReferenceExist(ctx context.Context, repoPath, name string) bool {
 	_, _, err := NewCommand(ctx, "show-ref", "--verify").AddDashesAndList(name).RunStdString(&RunOpts{Dir: repoPath})