Add action to push (mirror) changes to a different repository (OSOE-737)

[BenedekFarkas]

For example to sync changes from our working repository of a project to release to the client's. Check if there are suitable actions in the GitHub Marketplace to wrap, e.g., https://github.com/marketplace/actions/mirror-repository.

Empty git pushes must be prevented by checking the state of the target remote with git log, also see Lombiq/Git-hg-Mirror-Daemon#2.

Jira issue

[Piedone]

Git-hg Mirror only pushes to the target repo if there are changes. This needs to retain that functionality, because unnecessary git pushes are frowned upon by GitHub.

[BenedekFarkas]

Updated description with the link to the GHM issue where we discussed this.