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.
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
$ cd ..