repo: When writing to a non-root owned repo, chown() objects to match

Some package systems need to be run as root, so the process linking to
libostree may also be root.  However, it's reasonable to have the
target repository be owned by a uid other than root.

This patch makes it Just Work by chowning the file content to match.

Note this only operates on archive-z2 repositories, because you can't
usefully serve bare repositories via HTTP.

https://bugzilla.gnome.org/show_bug.cgi?id=738954