core: Support for committing from any libarchive-supported format

core: Support for committing from any libarchive-supported format

We really want the ability to take a .tar.gz and directly import
it into a repository, without creating a temporary filesystem tree.

First, doing it this way is significantly faster.  Also, this allows
us to handle importing tar files with e.g. uid 0 files into packed
repositories as non-root, which is very useful for tests and builds.