Move GetDiverging functions to gitrepo (#35524)

Extracted from #35469

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/gitrepo/main_test.go

@@ -0,0 +1,32 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gitrepo
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/tempdir"
+	"code.gitea.io/gitea/modules/test"
+)
+
+func TestMain(m *testing.M) {
+	gitHomePath, cleanup, err := tempdir.OsTempDir("gitea-test").MkdirTempRandom("git-home")
+	if err != nil {
+		log.Fatal("Unable to create temp dir: %v", err)
+	}
+	defer cleanup()
+
+	// resolve repository path relative to the test directory
+	testRootDir := test.SetupGiteaRoot()
+	repoPath = func(repo Repository) string {
+		return filepath.Join(testRootDir, "/modules/git/tests/repos", repo.RelativePath())
+	}
+
+	setting.Git.HomePath = gitHomePath
+	os.Exit(m.Run())
+}