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>

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
+```