Add agit flow support in gitea (#14295)

* feature: add agit flow support

ref: https://git-repo.info/en/2020/03/agit-flow-and-git-repo/

example:

```Bash
git checkout -b test
echo "test" >> README.md
git commit -m "test"
git push origin HEAD:refs/for/master -o topic=test
```

Signed-off-by: a1012112796 <1012112796@qq.com>

* fix lint

* simplify code add fix some nits

* update merge help message

* Apply suggestions from code review. Thanks @jiangxin

* add forced-update message

* fix lint

* splite writePktLine

* add refs/for/<target-branch>/<topic-branch> support also

* Add test code add fix api

* fix lint

* fix test

* skip test if git version < 2.29

* try test with git 2.30.1

* fix permission check bug

* fix some nit

* logic implify and test code update

* fix bug

* apply suggestions from code review

* prepare for merge

Signed-off-by: Andrew Thornton <art27@cantab.net>

* fix permission check bug

- test code update
- apply suggestions from code review @zeripath

Signed-off-by: a1012112796 <1012112796@qq.com>

* fix bug when target branch isn't exist

* prevent some special push and fix some nits

* fix lint

* try splite

* Apply suggestions from code review

- fix permission check
- handle user rename

* fix version negotiation

* remane

* fix template

* handle empty repo

* ui: fix  branch link under the title

* fix nits

Co-authored-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

routers/web/repo/pull.go

@@ -427,10 +427,18 @@ func PrepareViewPullInfo(ctx *context.Context, issue *models.Issue) *git.Compare
 		}
 		defer headGitRepo.Close()
 
-		headBranchExist = headGitRepo.IsBranchExist(pull.HeadBranch)
+		if pull.Flow == models.PullRequestFlowGithub {
+			headBranchExist = headGitRepo.IsBranchExist(pull.HeadBranch)
+		} else {
+			headBranchExist = git.IsReferenceExist(baseGitRepo.Path, pull.GetGitRefName())
+		}
 
 		if headBranchExist {
-			headBranchSha, err = headGitRepo.GetBranchCommitID(pull.HeadBranch)
+			if pull.Flow != models.PullRequestFlowGithub {
+				headBranchSha, err = baseGitRepo.GetRefCommitID(pull.GetGitRefName())
+			} else {
+				headBranchSha, err = headGitRepo.GetBranchCommitID(pull.HeadBranch)
+			}
 			if err != nil {
 				ctx.ServerError("GetBranchCommitID", err)
 				return nil