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>

models/pull_test.go

@@ -92,11 +92,11 @@ func TestPullRequestsOldest(t *testing.T) {
 
 func TestGetUnmergedPullRequest(t *testing.T) {
 	assert.NoError(t, PrepareTestDatabase())
-	pr, err := GetUnmergedPullRequest(1, 1, "branch2", "master")
+	pr, err := GetUnmergedPullRequest(1, 1, "branch2", "master", PullRequestFlowGithub)
 	assert.NoError(t, err)
 	assert.Equal(t, int64(2), pr.ID)
 
-	_, err = GetUnmergedPullRequest(1, 9223372036854775807, "branch1", "master")
+	_, err = GetUnmergedPullRequest(1, 9223372036854775807, "branch1", "master", PullRequestFlowGithub)
 	assert.Error(t, err)
 	assert.True(t, IsErrPullRequestNotExist(err))
 }