Allow any ref, not just tags or branches

[mattwynne]

The current regex for parsing a receive-pack payload assumes the ref will be of the form:

refs/tags/:tag-name
refs/heads/:branch-name

We'd like to be able to push arbitrary refs, like

refs/any/thing/we/like

The goal of this PR is to make that change, with tests and any neccesary changes to the outside API / internal model.

To do:

• [ ] fix the regex so it works for any number of slashes/segments (currently assumes the form refs/one/two)
• [ ] tests / API changes

[gabrielcsapo]

This is great, any updates on tests @mattwynne ?

[mattwynne]

I'm afraid I haven't had any time to give this attention since I shared my spike code. I got stuck on #81 - because I want to trust my environment before I start adding more tests.

[gabrielcsapo]

I can unblock you on that this weekend!

[gabrielcsapo]

@mattwynne tests should be fixed, CI is now running via GitHub actions!