Some refactors about GetMergeBase (#36186)

Maybe fix #32018

- Use `gitrepo.GetMergeBase` method instead of other two
implementations.
- Add `FetchRemoteCommit` so that we don't need to add many `remote` to
the git repository to avoid possible git lock conflicts. A lock will
start when invoke the function, it will be invoked when cross-repository
comparing. The head repository will fetch the base repository's base
commit id. In most situations, it should lock the fork repositories so
that it should not become a bottleneck.
- Improve `GetCompareInfo` to remove unnecessarily adding remote.
- Remove unnecessary parameters of `SignMerge`.

services/migrations/gitea_uploader.go

@@ -732,7 +732,7 @@ func (g *GiteaLocalUploader) newPullRequest(ctx context.Context, pr *base.PullRe
 		if pr.Base.Ref != "" && pr.Head.SHA != "" {
 			// A PR against a tag base does not make sense - therefore pr.Base.Ref must be a branch
 			// TODO: should we be checking for the refs/heads/ prefix on the pr.Base.Ref? (i.e. are these actually branches or refs)
-			pr.Base.SHA, _, err = g.gitRepo.GetMergeBase("", git.BranchPrefix+pr.Base.Ref, pr.Head.SHA)
+			pr.Base.SHA, err = gitrepo.MergeBase(ctx, g.repo, git.BranchPrefix+pr.Base.Ref, pr.Head.SHA)
 			if err != nil {
 				log.Error("Cannot determine the merge base for PR #%d in %s/%s. Error: %v", pr.Number, g.repoOwner, g.repoName, err)
 			}