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

tests/installed/itest-pull.sh

@@ -25,3 +25,17 @@ run_tmp_webserver $(pwd)/repo
 ostree --repo=bare-repo init --mode=bare-user
 ostree --repo=bare-repo remote add origin --set=gpg-verify=false $(cat ${test_tmpdir}/httpd-address)
 ostree --repo=bare-repo pull --disable-static-deltas origin ${host_nonremoteref}
+
+rm bare-repo repo -rf
+
+# Try copying the host's repo across a mountpoint for direct
+# imports.
+cd ${test_tmpdir}
+mkdir tmpfs mnt
+mount --bind tmpfs mnt
+cd mnt
+ostree --repo=repo init --mode=bare
+ostree --repo=repo pull-local /ostree/repo ${host_commit}
+ostree --repo=repo fsck
+cd ..
+umount mnt