Add Contributed backport command (#22643)

This PR provides a contributed backport command to help create backports
for Gitea.

It represents a significant improvement on my previously described
shell-script.

It can be installed using `go install contrib/backport/backport.go`.

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

Author: zeripath

Diff for: CONTRIBUTING.md

@@ -267,26 +267,10 @@ with the rest of the summary matching the original PR. Similarly for frontports
 
 ---
 
-The below is a script that may be helpful in creating backports. YMMV.
+A command to help create backports can be found in `contrib/backport` and can be installed (from inside the gitea repo root directory) using:
 
 ```bash
-#!/bin/sh
-PR="$1"
-SHA="$2"
-VERSION="$3"
-
-if [ -z "$SHA" ]; then
-    SHA=$(gh api /repos/go-gitea/gitea/pulls/$PR -q '.merge_commit_sha')
-fi
-
-if [ -z "$VERSION" ]; then
-    VERSION="v1.16"
-fi
-
-echo git checkout origin/release/"$VERSION" -b backport-$PR-$VERSION
-git checkout origin/release/"$VERSION" -b backport-$PR-$VERSION
-git cherry-pick $SHA && git commit --amend && git push zeripath backport-$PR-$VERSION && xdg-open https://github.com/go-gitea/gitea/compare/release/"$VERSION"...zeripath:backport-$PR-$VERSION
-
+go install contrib/backport/backport.go
 ```
 
 ## Developer Certificate of Origin (DCO)

Diff for: contrib/backport/README

@@ -0,0 +1,41 @@
+`backport`
+==========
+
+`backport` is a command to help create backports of PRs. It backports a
+provided PR from main on to a released version.
+
+It will create a backport branch, cherry-pick the PR's merge commit, adjust
+the commit message and then push this back up to your fork's remote.
+
+The default version will read from `docs/config.yml`. You can override this
+using the option `--version`.
+
+The upstream branches will be fetched, using the remote `origin`. This can
+be overrided using `--upstream`, and fetching can be avoided using
+`--no-fetch`.
+
+By default the branch created will be called `backport-$PR-$VERSION`. You
+can override this using the option `--backport-branch`. This branch will
+be created from `--release-branch` which is `release/$(VERSION)`
+by default and will be pulled from `$(UPSTREAM)`.
+
+The merge-commit as determined by the github API will be used as the SHA to
+cherry-pick. You can override this using `--cherry-pick`.
+
+The commit message will be amended to add the `Backport` header.
+`--no-amend-message` can be set to stop this from happening.
+
+If cherry-pick is successful the backported branch will be pushed up to your
+fork using your remote. These will be determined using `git remote -v`. You
+can set your fork name using `--fork-user` and your remote name using
+`--remote`. You can avoid pushing using `--no-push`.
+
+If the push is successful, `xdg-open` will be called to open a backport url.
+You can stop this using `--no-xdg-open`.
+
+Installation
+============
+
+```bash
+go install contrib/backport/backport.go
+```