Support tags as well as shas

[asottile]

They kind-of work as-is unless tags collide across different hooks.

The simplest solution is to make the repository symlinks repo + ref instead of just ref as they are right now.

Then anything that git understands as git checkout {0} will work.

[asottile]

This is my start on this branch: https://github.com/pre-commit/pre-commit/tree/allow_tags

I don't think repo + ref is going to work because slashes, but maybe a hash of that will work.

[asottile]

Partially implemented with #127

Need to add ref and deprecate sha.

Probably should add some additional logic for tags into autoupdate. Should be reasonably easy to do a natural sort on tags and bump to the latest tag (instead of reverting back to shas). At the very least it should not bump a tag to a sha.

[asottile]

The autoupdate work was done here via:

• Make autoupdate --tags-only the default, add --bleeding-edge #529
• Add a --tags-only option to autoupdate #476

All that's left is to support ref and deprecate sha

[asottile]

This has been released as part of v1.7.0!