rofiles: Add --copyup option

Sadly https://sourceware.org/bugzilla/show_bug.cgi?id=22089 is I think going to
actually force us to cave here. Even if we got the glibc patch in today, we need
to support the RHEL glibc. See also discussion about fish as part of the general
Fedora tracker.

This is basically needed to unblock rpm-ostree unified core 🌐:
https://github.com/projectatomic/rpm-ostree/issues/729

Closes: https://github.com/ostreedev/ostree/issues/1377

Closes: #1382
Approved by: jlebon

tests/test-rofiles-fuse.sh

@@ -26,7 +26,7 @@ skip_without_user_xattrs
 
 setup_test_repository "bare"
 
-echo "1..8"
+echo "1..11"
 
 cd ${test_tmpdir}
 mkdir mnt
@@ -117,3 +117,23 @@ echo "ok checkout copy fallback"
 
 # check that O_RDONLY|O_CREAT is handled correctly; used by flock(1) at least
 flock mnt/nonexistent-file echo "ok create file in ro mode"
+echo "ok flock"
+
+# And now with --copyup enabled
+
+fusermount -u ${test_tmpdir}/mnt
+assert_not_has_file mnt/firstfile
+rofiles-fuse --copyup checkout-test2 mnt
+assert_file_has_content mnt/firstfile first
+echo "ok copyup mount"
+
+firstfile_orig_inode=$(stat -c %i checkout-test2/firstfile)
+for path in firstfile{,-link}; do
+    echo truncating > mnt/${path}
+    assert_file_has_content mnt/${path} truncating
+    assert_not_file_has_content mnt/${path} first
+done
+firstfile_new_inode=$(stat -c %i checkout-test2/firstfile)
+assert_not_streq "${firstfile_orig_inode}" "${firstfile_new_inode}"
+
+echo "ok copyup"