Move GetDiverging functions to gitrepo (#35524)

Extracted from #35469

---------

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

modules/gitrepo/compare_test.go

@@ -0,0 +1,42 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gitrepo
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+type mockRepository struct {
+	path string
+}
+
+func (r *mockRepository) RelativePath() string {
+	return r.path
+}
+
+func TestRepoGetDivergingCommits(t *testing.T) {
+	repo := &mockRepository{path: "repo1_bare"}
+	do, err := GetDivergingCommits(t.Context(), repo, "master", "branch2")
+	assert.NoError(t, err)
+	assert.Equal(t, &DivergeObject{
+		Ahead:  1,
+		Behind: 5,
+	}, do)
+
+	do, err = GetDivergingCommits(t.Context(), repo, "master", "master")
+	assert.NoError(t, err)
+	assert.Equal(t, &DivergeObject{
+		Ahead:  0,
+		Behind: 0,
+	}, do)
+
+	do, err = GetDivergingCommits(t.Context(), repo, "master", "test")
+	assert.NoError(t, err)
+	assert.Equal(t, &DivergeObject{
+		Ahead:  0,
+		Behind: 2,
+	}, do)
+}