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>

cmd/serv.go

@@ -17,6 +17,7 @@ import (
 	"time"
 
 	"code.gitea.io/gitea/models"
+	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/json"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/pprof"
@@ -146,6 +147,13 @@ func runServ(c *cli.Context) error {
 	}
 
 	if len(words) < 2 {
+		if git.CheckGitVersionAtLeast("2.29") == nil {
+			// for AGit Flow
+			if cmd == "ssh_info" {
+				fmt.Print(`{"type":"gitea","version":1}`)
+				return nil
+			}
+		}
 		return fail("Too few arguments", "Too few arguments in cmd: %s", cmd)
 	}