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`.

routers/web/repo/issue_view.go

@@ -495,7 +495,7 @@ func preparePullViewSigning(ctx *context.Context, issue *issues_model.Issue) {
 	pull := issue.PullRequest
 	ctx.Data["WillSign"] = false
 	if ctx.Doer != nil {
-		sign, key, _, err := asymkey_service.SignMerge(ctx, pull, ctx.Doer, ctx.Repo.GitRepo, pull.BaseBranch, pull.GetGitHeadRefName())
+		sign, key, _, err := asymkey_service.SignMerge(ctx, pull, ctx.Doer, ctx.Repo.GitRepo)
 		ctx.Data["WillSign"] = sign
 		ctx.Data["SigningKeyMergeDisplay"] = asymkey_model.GetDisplaySigningKey(key)
 		if err != nil {