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/pull/check.go

@@ -238,7 +238,7 @@ func isSignedIfRequired(ctx context.Context, pr *issues_model.PullRequest, doer
 	}
 	defer closer.Close()
 
-	sign, _, _, err := asymkey_service.SignMerge(ctx, pr, doer, gitRepo, pr.BaseBranch, pr.GetGitHeadRefName())
+	sign, _, _, err := asymkey_service.SignMerge(ctx, pr, doer, gitRepo)
 
 	return sign, err
 }