remotefilelog: log when we're about to fetch files
I'm debugging a slow client situation and knowing how many files are
in the batch request would be a nice thing.
Differential Revision: https://phab.mercurial-scm.org/D6353
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state