Support for git repos using reftable backend

[sandyarmstrong]

reftable is a (technically experimental) refs backend for git that has recently started outperforming the default backend for very large repositories. It is expected to become the default in git 3.0.

Clone a repo that uses SourceLink targets using git clone --ref-format reftable, and you will get the following error at build-time.

Unsupported repository extension 'refstorage'

This is a similar error to #1097 and #1468.

I created #1469 as a naive way to get past this error, but it turns out SourceLink actually needs to read refs.

So it looks like SourceLink will need to implement the ability to read this new binary format. The spec is available at https://git-scm.com/docs/reftable, and this good reference article was written when it was added to git last August.