refactor: make git package handle all git operations (#38543)

before: gitrepo vs git packages
after: git package fully handle all git operations

by the way, use `WithRepo(repo)` instead of `WithDir(repo.Path)` to hide
path details.

benefits:

1. remove all unnecessary wrappers, developers no need to struggle with
"which package should be used"
2. simplify code, RepositoryFacade can (will) be used everywhere, all
"path" details are (will be) hidden

modules/git/merge.go

@@ -0,0 +1,26 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package git
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"gitea.dev/modules/git/gitcmd"
+	"gitea.dev/modules/util"
+)
+
+// MergeBase checks and returns merge base of two commits.
+func MergeBase(ctx context.Context, repo RepositoryFacade, baseCommitID, headCommitID string) (string, error) {
+	mergeBase, stderr, err := gitcmd.NewCommand("merge-base").
+		AddDashesAndList(baseCommitID, headCommitID).WithRepo(repo).RunStdString(ctx)
+	if err != nil {
+		if gitcmd.IsErrorExitCode(err, 1) && strings.TrimSpace(stderr) == "" {
+			return "", util.NewNotExistErrorf("merge-base for %s and %s doesn't exist", baseCommitID, headCommitID)
+		}
+		return "", fmt.Errorf("get merge-base of %s and %s failed: %w", baseCommitID, headCommitID, err)
+	}
+	return strings.TrimSpace(mergeBase), nil
+}