Add API endpoints to tinker with git references

[tle-huu]

GitHub does have some API endpoints to create, update, or get references: https://developer.github.com/v3/git/refs/

These endpoints can be useful for users to recreate some git "plumbing" functionalities over the Gitea API. With these endpoints, users could reset branches for example.

More generally, GitHub has a "Git Database API" which works at a lower level and allows user to do almost anything, but that might be a bigger project: https://developer.github.com/v3/git/

[stale]

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[stale]

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[richmahn]

Seems to be the same as #16208

Github doesn't even have a create branch API endpoint like Gitea has, but just relies on a covers-all create reference API end point:

https://docs.github.com/en/rest/git/refs#create-a-reference

This and the update reference are needed.

[richmahn]

Currently still working on this. Writing unit tests.