Support "merge upstream branch" (Sync fork) (#32741)

Add basic "sync fork" support (GitHub-like)

<details>

![image](https://github.com/user-attachments/assets/e71473f4-4518-48c7-b9e2-fedfcd564fc3)

</details>

services/pull/update.go

@@ -65,7 +65,9 @@ func Update(ctx context.Context, pr *issues_model.PullRequest, doer *user_model.
 		return fmt.Errorf("unable to load HeadRepo for PR[%d] during update-by-merge: %w", pr.ID, err)
 	}
 
-	// use merge functions but switch repos and branches
+	// TODO: FakePR: it is somewhat hacky, but it is the only way to "merge" at the moment
+	// ideally in the future the "merge" functions should be refactored to decouple from the PullRequest
+	// now use a fake reverse PR to switch head&base repos/branches
 	reversePR := &issues_model.PullRequest{
 		ID: pr.ID,
 

services/repository/merge_upstream.go

@@ -0,0 +1,115 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package repository
+
+import (
+	"context"
+	"fmt"
+
+	git_model "code.gitea.io/gitea/models/git"
+	issue_model "code.gitea.io/gitea/models/issues"
+	repo_model "code.gitea.io/gitea/models/repo"
+	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/git"
+	repo_module "code.gitea.io/gitea/modules/repository"
+	"code.gitea.io/gitea/modules/util"
+	"code.gitea.io/gitea/services/pull"
+)
+
+type UpstreamDivergingInfo struct {
+	BaseIsNewer   bool
+	CommitsBehind int
+	CommitsAhead  int
+}
+
+func MergeUpstream(ctx context.Context, doer *user_model.User, repo *repo_model.Repository, branch string) (mergeStyle string, err error) {
+	if err = repo.MustNotBeArchived(); err != nil {
+		return "", err
+	}
+	if err = repo.GetBaseRepo(ctx); 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),
+	})
+	if err == nil {
+		return "fast-forward", nil
+	}
+	if !git.IsErrPushOutOfDate(err) && !git.IsErrPushRejected(err) {
+		return "", err
+	}
+
+	// TODO: FakePR: it is somewhat hacky, but it is the only way to "merge" at the moment
+	// ideally in the future the "merge" functions should be refactored to decouple from the PullRequest
+	fakeIssue := &issue_model.Issue{
+		ID:       -1,
+		RepoID:   repo.ID,
+		Repo:     repo,
+		Index:    -1,
+		PosterID: doer.ID,
+		Poster:   doer,
+		IsPull:   true,
+	}
+	fakePR := &issue_model.PullRequest{
+		ID:         -1,
+		Status:     issue_model.PullRequestStatusMergeable,
+		IssueID:    -1,
+		Issue:      fakeIssue,
+		Index:      -1,
+		HeadRepoID: repo.ID,
+		HeadRepo:   repo,
+		BaseRepoID: repo.BaseRepo.ID,
+		BaseRepo:   repo.BaseRepo,
+		HeadBranch: branch, // maybe HeadCommitID is not needed
+		BaseBranch: branch,
+	}
+	fakeIssue.PullRequest = fakePR
+	err = pull.Update(ctx, fakePR, doer, "merge upstream", false)
+	if err != nil {
+		return "", err
+	}
+	return "merge", nil
+}
+
+func GetUpstreamDivergingInfo(ctx context.Context, repo *repo_model.Repository, branch string) (*UpstreamDivergingInfo, error) {
+	if !repo.IsFork {
+		return nil, util.NewInvalidArgumentErrorf("repo is not a fork")
+	}
+
+	if repo.IsArchived {
+		return nil, util.NewInvalidArgumentErrorf("repo is archived")
+	}
+
+	if err := repo.GetBaseRepo(ctx); err != nil {
+		return nil, err
+	}
+
+	forkBranch, err := git_model.GetBranch(ctx, repo.ID, branch)
+	if err != nil {
+		return nil, err
+	}
+
+	baseBranch, err := git_model.GetBranch(ctx, repo.BaseRepo.ID, branch)
+	if err != nil {
+		return nil, err
+	}
+
+	info := &UpstreamDivergingInfo{}
+	if forkBranch.CommitID == baseBranch.CommitID {
+		return info, nil
+	}
+
+	// TODO: if the fork repo has new commits, this call will fail:
+	// exit status 128 - fatal: Invalid symmetric difference expression aaaaaaaaaaaa...bbbbbbbbbbbb
+	// so at the moment, we are not able to handle this case, should be improved in the future
+	diff, err := git.GetDivergingCommits(ctx, repo.BaseRepo.RepoPath(), baseBranch.CommitID, forkBranch.CommitID)
+	if err != nil {
+		info.BaseIsNewer = baseBranch.UpdatedUnix > forkBranch.UpdatedUnix
+		return info, nil
+	}
+	info.CommitsBehind, info.CommitsAhead = diff.Behind, diff.Ahead
+	return info, nil
+}