remotefilelog: fix bug in maybesparsematch returning alwaysmatcher
The description of the method says that it should return None if sparse is not
used in this repository; since sparse.matcher() returns alwaysmatcher if sparse
is not enabled, I'm using that as the signal to return None here to preserve the
previous behavior.
Differential Revision: https://phab.mercurial-scm.org/D5487