path_auditor: cache names of audited directories
We use a separate cache to avoid problems with
audit = path_auditor(repo.root)
audit("subrepo")
audit("subrepo/file")
whitelisting "subrepo" (which is fine) and then using the same whitelist
with "subrepo/file" (which is not fine).
Since we create a separate path_auditor for every path on the command line,
a "hg add dir/a dir/b dir/c" will still lstat dir 3 times just to audit
the paths.