Improve source-repositories git caching.

We now unconditionally cache the repositories being fetched with
source-repositories.

Additionally, by improving the way we handle repository refs, we are now
able to build images with code taken directly from gerrit reviews.

Change-Id: Ifd4c5691f6761eb5551663e6d0aa1c0e42afced3
Log unsupported source repository types.

If we don't recognise git/tar/etc as source-repositories repotypes, we
now log the repotype, for debugging purposes.

Change-Id: I7b870807658441c928c7880291eb0284b6f97778