Add git-obs subcommand for reparenting a git repo

[dmach]

Sometimes, we need to create a copy of an existing repo in pool.
In that case, we need to make the new repo a parent to the existing repo, which means creating the for with reverse parent relation than usual.

An example command:
git-obs api -X POST --data '{ "organization": "pool", "reparent": true }' /repos/<owner>/<repo>/forks

This requires admin owner/admin perms in both source and target organizations.
In short: this should be ideally performed by the server admin.

It seems to be a custom patch, upstream will probably implement it differently: go-gitea/gitea#35430