remove: use ctx.hasdir(f) instead of 'f in ctx.dirs()'
This speeds up 'hg remove python/README' on the Firefox repo from
2.479s to 0.664s with lazily loaded treemanifests (which is not yet in
core) and has no measurable effect on flat manifests.
$ 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