don't update sync time if no files have changed (and thus avoid re-indexing after every sync attempt)
don't update sync time if no files have changed
(and thus avoid re-indexing after every sync attempt)

R=rsc
http://go/go-review/1016010
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go