libostree: set directory mtimes to 0 on checkout

We already set all file mtimes to 0 so that they are constant
over all checkouts, and can be made constant with a known value from
the system where the ostree was created.

However, this was not happening for directories. Zero their mtimes too.

This is important for shipping a fontconfig cache in the ostree;
the fontconfig cache files embed a directory mtime.

tests/basic-test.sh

@@ -347,6 +347,8 @@ else
 fi
 stat '--format=%Y' test2-checkout/baz/cow > cow-mtime
 assert_file_has_content cow-mtime 0
+stat '--format=%Y' test2-checkout/baz/deeper > deeper-mtime
+assert_file_has_content deeper-mtime 0
 echo "ok content mtime"
 
 cd ${test_tmpdir}