pull: Copy the upstream summary file when doing a pull --mirror

While it could be regenerated downstream, there might be other
metadata upstream, and the goal here is a mirror.

https://bugzilla.gnome.org/show_bug.cgi?id=739377

tests/test-pull-mirror-summary.sh

@@ -41,6 +41,7 @@ mkdir repo
 ostree --repo=repo init --mode=archive-z2
 ostree --repo=repo remote add --set=gpg-verify=false origin $(cat httpd-address)/ostree/gnomerepo
 ostree --repo=repo pull --mirror origin
+assert_has_file repo/summary
 ostree --repo=repo checkout -U main main-copy
 assert_file_has_content main-copy/baz/cow "moo"
 ostree --repo=repo checkout -U other other-copy