bug#66364: 29.1.50; vc-hg-registered/state are slow on directories of large repos

[Spencer Baugh]

vc-hg-state (and vc-hg-registered, which calls vc-hg-state) are slow
when run on directories of large repos.  In fact they're O(N) in the
number of files in the repo.

This is because vc-hg-state-slow runs "hg status" on directories in a
mode which lists all the files in the directory, and then it parses that
list.  Which is pointlessly slow.

However, Hg (like git) does not actually track directories.  So in the
end vc-hg-state on a directory should always be returning 'unregistered,
which matches the existing behavior of vc-hg (vc-hg-registered always
returned nil for directories) and is much faster.  (It also matches what
vc-git does.)

A patch to do this will follow