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/web.go

@@ -12,6 +12,7 @@ import (
 
 	"code.gitea.io/gitea/models"
 	"code.gitea.io/gitea/modules/context"
+	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/httpcache"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/metrics"
@@ -146,6 +147,21 @@ func Routes() *web.Route {
 		routes.Get("/metrics", append(common, Metrics)...)
 	}
 
+	routes.Get("/ssh_info", func(rw http.ResponseWriter, req *http.Request) {
+		if !git.SupportProcReceive {
+			rw.WriteHeader(404)
+			return
+		}
+		rw.Header().Set("content-type", "text/json;charset=UTF-8")
+		_, err := rw.Write([]byte(`{"type":"gitea","version":1}`))
+		if err != nil {
+			log.Error("fail to write result: err: %v", err)
+			rw.WriteHeader(500)
+			return
+		}
+		rw.WriteHeader(200)
+	})
+
 	// Removed: toolbox.Toolboxer middleware will provide debug information which seems unnecessary
 	common = append(common, context.Contexter())