Move GetDiverging functions to gitrepo (#35524)

Extracted from #35469

---------

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

modules/gitrepo/compare.go

@@ -0,0 +1,44 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gitrepo
+
+import (
+	"context"
+	"fmt"
+	"strconv"
+	"strings"
+
+	"code.gitea.io/gitea/modules/git/gitcmd"
+)
+
+// DivergeObject represents commit count diverging commits
+type DivergeObject struct {
+	Ahead  int
+	Behind int
+}
+
+// GetDivergingCommits returns the number of commits a targetBranch is ahead or behind a baseBranch
+func GetDivergingCommits(ctx context.Context, repo Repository, baseBranch, targetBranch string) (*DivergeObject, error) {
+	cmd := gitcmd.NewCommand("rev-list", "--count", "--left-right").
+		AddDynamicArguments(baseBranch + "..." + targetBranch).AddArguments("--")
+	stdout, _, err1 := cmd.RunStdString(ctx, &gitcmd.RunOpts{Dir: repoPath(repo)})
+	if err1 != nil {
+		return nil, err1
+	}
+
+	left, right, found := strings.Cut(strings.Trim(stdout, "\n"), "\t")
+	if !found {
+		return nil, fmt.Errorf("git rev-list output is missing a tab: %q", stdout)
+	}
+
+	behind, err := strconv.Atoi(left)
+	if err != nil {
+		return nil, err
+	}
+	ahead, err := strconv.Atoi(right)
+	if err != nil {
+		return nil, err
+	}
+	return &DivergeObject{Ahead: ahead, Behind: behind}, nil
+}