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.

mercurial/cmdutil.py

@@ -2336,7 +2336,7 def remove(ui, repo, m, prefix, after, f
                     return True
             return False
 
-        isdir = f in deleteddirs or f in wctx.dirs()
+        isdir = f in deleteddirs or wctx.hasdir(f)
         if f in repo.dirstate or isdir or f == '.' or insubrepo():
             continue