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/pull.go

@@ -521,14 +521,7 @@ func checkIfPRContentChanged(ctx context.Context, pr *issues_model.PullRequest,
 	}
 	defer cancel()
 
-	tmpRepo, err := git.OpenRepository(ctx, prCtx.tmpBasePath)
-	if err != nil {
-		return false, "", fmt.Errorf("OpenRepository: %w", err)
-	}
-	defer tmpRepo.Close()
-
-	// Find the merge-base
-	mergeBase, _, err = tmpRepo.GetMergeBase("", "base", "tracking")
+	mergeBase, err = gitrepo.MergeBase(ctx, pr.BaseRepo, pr.BaseBranch, pr.GetGitHeadRefName())
 	if err != nil {
 		return false, "", fmt.Errorf("GetMergeBase: %w", err)
 	}