pull: Support --mirror option

There's several use cases for calling into ostree itself to do
mirroring, instead of using bare rsync.  For example, it's a bit more
efficient as it doesn't require syncing the objects/ directory.

https://bugzilla.gnome.org/show_bug.cgi?id=728351

tests/pull-test.sh

@@ -36,6 +36,15 @@ assert_file_has_content firstfile '^first$'
 assert_file_has_content baz/cow '^moo$'
 echo "ok pull contents"
 
+cd ${test_tmpdir}
+mkdir mirrorrepo
+ostree --repo=mirrorrepo init --mode=archive-z2
+${CMD_PREFIX} ostree --repo=mirrorrepo remote add --set=gpg-verify=false origin $(cat httpd-address)/ostree/gnomerepo
+${CMD_PREFIX} ostree --repo=mirrorrepo pull --mirror origin main
+${CMD_PREFIX} ostree --repo=mirrorrepo fsck
+$OSTREE show main >/dev/null
+echo "ok pull mirror"
+
 cd ${test_tmpdir}
 ostree --repo=ostree-srv/gnomerepo commit -b main -s "Metadata string" --add-detached-metadata-string=SIGNATURE=HANCOCK --tree=ref=main
 ${CMD_PREFIX} ostree --repo=repo pull origin main