dirstate-tree: Skip readdir() in `hg status -mard` When running the status algorithm in a mode where we don’t list unknown or ignored files, all we care about are files that are listed in the dirstate. We can there for skip making expensive calls to readdir() to list the contents of filesystem directories, and instead only run stat() to get the filesystem state of files listed in the dirstate. (This state may be an error for files that don’t exist anymore on the filesystem.) On 16 CPU threads, this reduces the time spent in the `status()` function for `hg status -mard` on an old snapshot of mozilla-central from ~70ms to ~50ms. Differential Revision: https://phab.mercurial-scm.org/D10752

# Files that just need to be migrated to the formatter.
# Do not add new files here!
mercurial/cext/manifest.c
mercurial/cext/osutil.c
# Vendored code that we should never format:
syntax: glob
contrib/python-zstandard/**.c
contrib/python-zstandard/**.h
hgext/fsmonitor/pywatchman/**.c
mercurial/thirdparty/**.c
mercurial/thirdparty/**.h
mercurial/pythoncapi_compat.h