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`.

modules/git/repo_compare.go

@@ -18,32 +18,6 @@ import (
 	"code.gitea.io/gitea/modules/git/gitcmd"
 )
 
-// GetMergeBase checks and returns merge base of two branches and the reference used as base.
-func (repo *Repository) GetMergeBase(tmpRemote, base, head string) (string, string, error) {
-	if tmpRemote == "" {
-		tmpRemote = "origin"
-	}
-
-	if tmpRemote != "origin" {
-		tmpBaseName := RemotePrefix + tmpRemote + "/tmp_" + base
-		// Fetch commit into a temporary branch in order to be able to handle commits and tags
-		_, _, err := gitcmd.NewCommand("fetch", "--no-tags").
-			AddDynamicArguments(tmpRemote).
-			AddDashesAndList(base + ":" + tmpBaseName).
-			WithDir(repo.Path).
-			RunStdString(repo.Ctx)
-		if err == nil {
-			base = tmpBaseName
-		}
-	}
-
-	stdout, _, err := gitcmd.NewCommand("merge-base").
-		AddDashesAndList(base, head).
-		WithDir(repo.Path).
-		RunStdString(repo.Ctx)
-	return strings.TrimSpace(stdout), base, err
-}
-
 type lineCountWriter struct {
 	numLines int
 }