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
 }

modules/gitrepo/fetch.go

@@ -0,0 +1,28 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gitrepo
+
+import (
+	"context"
+
+	"code.gitea.io/gitea/modules/git/gitcmd"
+	"code.gitea.io/gitea/modules/globallock"
+)
+
+// FetchRemoteCommit fetches a specific commit and its related objects from a remote
+// repository into the managed repository.
+//
+// If no reference (branch, tag, or other ref) points to the fetched commit, it will
+// be treated as unreachable and cleaned up by `git gc` after the default prune
+// expiration period (2 weeks). Ref: https://www.kernel.org/pub/software/scm/git/docs/git-gc.html
+//
+// This behavior is sufficient for temporary operations, such as determining the
+// merge base between commits.
+func FetchRemoteCommit(ctx context.Context, repo, remoteRepo Repository, commitID string) error {
+	return globallock.LockAndDo(ctx, getRepoWriteLockKey(repo.RelativePath()), func(ctx context.Context) error {
+		return RunCmd(ctx, repo, gitcmd.NewCommand("fetch", "--no-tags").
+			AddDynamicArguments(repoPath(remoteRepo)).
+			AddDynamicArguments(commitID))
+	})
+}

modules/gitrepo/merge.go

@@ -0,0 +1,22 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gitrepo
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"code.gitea.io/gitea/modules/git/gitcmd"
+)
+
+// MergeBase checks and returns merge base of two commits.
+func MergeBase(ctx context.Context, repo Repository, baseCommitID, headCommitID string) (string, error) {
+	mergeBase, err := RunCmdString(ctx, repo, gitcmd.NewCommand("merge-base").
+		AddDashesAndList(baseCommitID, headCommitID))
+	if err != nil {
+		return "", fmt.Errorf("get merge-base of %s and %s failed: %w", baseCommitID, headCommitID, err)
+	}
+	return strings.TrimSpace(mergeBase), nil
+}

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
+}