feat: sync forks (#2364)

This allows syncing a branch of a fork with a branch of the base repo. It looks like this:
![grafik](/attachments/4508920c-7d0b-4330-9083-e3048733e38d)
This is only possible, if the fork don't have commits that are not in the main repo.

The feature is already working, but it is missing Finetuning, a better API, translations and tests, so this is currently WIP. It is also not tested with go-git.

<!--start release-notes-assistant-->

## Release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Features
  - [PR](https://codeberg.org/forgejo/forgejo/pulls/2364): <!--number 2364 --><!--line 0 --><!--description c3luYyBmb3Jrcw==-->sync forks<!--description-->
<!--end release-notes-assistant-->

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/2364
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: JakobDev <jakobdev@gmx.de>
Co-committed-by: JakobDev <jakobdev@gmx.de>

services/repository/sync_fork.go

@@ -0,0 +1,113 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package repository
+
+import (
+	"context"
+	"fmt"
+	"slices"
+
+	git_model "forgejo.org/models/git"
+	repo_model "forgejo.org/models/repo"
+	user_model "forgejo.org/models/user"
+	"forgejo.org/modules/git"
+	repo_module "forgejo.org/modules/repository"
+	api "forgejo.org/modules/structs"
+)
+
+// SyncFork syncs a branch of a fork with the base repo
+func SyncFork(ctx context.Context, doer *user_model.User, repo *repo_model.Repository, branch string) error {
+	err := repo.MustNotBeArchived()
+	if err != nil {
+		return err
+	}
+
+	err = repo.GetBaseRepo(ctx)
+	if err != nil {
+		return err
+	}
+
+	err = git.Push(ctx, repo.BaseRepo.RepoPath(), git.PushOptions{
+		Remote: repo.RepoPath(),
+		Branch: fmt.Sprintf("%s:%s", branch, branch),
+		Env:    repo_module.PushingEnvironment(doer, repo),
+	})
+
+	return err
+}
+
+// CanSyncFork returns information about syncing a fork
+func GetSyncForkInfo(ctx context.Context, repo *repo_model.Repository, branch string) (*api.SyncForkInfo, error) {
+	info := new(api.SyncForkInfo)
+
+	if !repo.IsFork {
+		return info, nil
+	}
+
+	if repo.IsArchived {
+		return info, nil
+	}
+
+	err := repo.GetBaseRepo(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	forkBranch, err := git_model.GetBranch(ctx, repo.ID, branch)
+	if err != nil {
+		return nil, err
+	}
+
+	info.ForkCommit = forkBranch.CommitID
+
+	baseBranch, err := git_model.GetBranch(ctx, repo.BaseRepo.ID, branch)
+	if err != nil {
+		if git_model.IsErrBranchNotExist(err) {
+			// If the base repo don't have the branch, we don't need to continue
+			return info, nil
+		}
+		return nil, err
+	}
+
+	info.BaseCommit = baseBranch.CommitID
+
+	// If both branches has the same latest commit, we don't need to sync
+	if forkBranch.CommitID == baseBranch.CommitID {
+		return info, nil
+	}
+
+	// Check if the latest commit of the fork is also in the base
+	gitRepo, err := git.OpenRepository(ctx, repo.BaseRepo.RepoPath())
+	if err != nil {
+		return nil, err
+	}
+	defer gitRepo.Close()
+
+	commit, err := gitRepo.GetCommit(forkBranch.CommitID)
+	if err != nil {
+		if git.IsErrNotExist(err) {
+			return info, nil
+		}
+		return nil, err
+	}
+
+	branchList, err := commit.GetAllBranches()
+	if err != nil {
+		return nil, err
+	}
+
+	if !slices.Contains(branchList, branch) {
+		return info, nil
+	}
+
+	diff, err := git.GetDivergingCommits(ctx, repo.BaseRepo.RepoPath(), baseBranch.CommitID, forkBranch.CommitID)
+	if err != nil {
+		return nil, err
+	}
+
+	info.Allowed = true
+	info.CommitsBehind = diff.Behind
+
+	return info, nil
+}