lib/commit: Add a copy fastpath for imports

lib/commit: Add a copy fastpath for imports

This fixes up the last of the embarassing bits I saw from
the stack trace in:
https://github.com/ostreedev/ostree/issues/1184

We had a hardlink fast path, but that doesn't apply across
devices, which occurs in two notable cases:

 - Installer ISO with local repo
 - Tools like pungi that copy the repo to a local snapshot

Obviously there are a lot of subtleties here around things like the
bare-user-only conversions as well as exactly what data we copy. I think to get
better test coverage we may want to add `pull-local --no-hardlink` or so.

Closes: #1197
Approved by: jlebon