For many cases of commit, we can actually optimize things by simply "adopting"
the object rather than writing a new copy. For example, in rpm-ostree package
layering.
We can only make that optimization though if we take ownership of the file. This
commit hence adds an API where a caller tells us to do so. For now, that just
means we `unlink()` the files/dirs as we go, but we can now later add the
"adopt" optimization.
Closes: #1255
Approved by: jlebon
This was already supported by the commit modifier API, just needed to
expose it. This will also be used to test the libarchive API in a future
test.
Closes: #275
Approved by: cgwalters