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/hook_test.go

@@ -0,0 +1,41 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package cmd
+
+import (
+	"bufio"
+	"bytes"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestPktLine(t *testing.T) {
+	// test read
+	s := strings.NewReader("0000")
+	r := bufio.NewReader(s)
+	result, err := readPktLine(r, pktLineTypeFlush)
+	assert.NoError(t, err)
+	assert.Equal(t, pktLineTypeFlush, result.Type)
+
+	s = strings.NewReader("0006a\n")
+	r = bufio.NewReader(s)
+	result, err = readPktLine(r, pktLineTypeData)
+	assert.NoError(t, err)
+	assert.Equal(t, pktLineTypeData, result.Type)
+	assert.Equal(t, []byte("a\n"), result.Data)
+
+	// test write
+	w := bytes.NewBuffer([]byte{})
+	err = writeFlushPktLine(w)
+	assert.NoError(t, err)
+	assert.Equal(t, []byte("0000"), w.Bytes())
+
+	w.Reset()
+	err = writeDataPktLine(w, []byte("a\nb"))
+	assert.NoError(t, err)
+	assert.Equal(t, []byte("0007a\nb"), w.Bytes())
+}