Do not read or write git reference files directly (#18079)

Git will and can pack references into packfiles and therefore if you write/read the
files directly you will get false results. Instead you should use update-ref and
show-ref. To that end I have created three new functions in git/repo_commit.go that
will do this correctly.

Related #17191

Signed-off-by: Andrew Thornton <art27@cantab.net>

services/migrations/gitea_uploader.go

@@ -698,8 +698,7 @@ func (g *GiteaLocalUploader) newPullRequest(pr *base.PullRequest) (*models.PullR
 			if pr.Head.SHA != "" {
 				// Git update-ref remove bad references with a relative path
 				log.Warn("Deprecated local head, removing : %v", pr.Head.SHA)
-				relPath := pr.GetGitRefName()
-				_, err = git.NewCommand("update-ref", "--no-deref", "-d", relPath).RunInDir(g.repo.RepoPath())
+				err = g.gitRepo.RemoveReference(pr.GetGitRefName())
 			} else {
 				// The SHA is empty, remove the head file
 				log.Warn("Empty reference, removing : %v", pullHead)