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/gitrepo/repo_lock.go

@@ -0,0 +1,10 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gitrepo
+
+// getRepoWriteLockKey returns the global lock key for write operations on the repository.
+// Parallel write operations on the same git repository should be avoided to prevent data corruption.
+func getRepoWriteLockKey(repoStoragePath string) string {
+	return "repo-write:" + repoStoragePath
+}