bug#62663: 28.2; vc-hg-state returns incorrect results on directories in some repos

[Spencer Baugh <sbaugh@janestreet.com>]

The problem is with vc-hg-state-fast, which parses Mercurial data
structures instead of shelling out to hg.  This is a great idea but our
parsing seems to be wrong right now in some way.

1. Create a new hg repo with:

hg init
  --config format.use_share_safe=false
  --config format.use-persistent-nodemap=false
  --config format.sparse-revlog=false
  --config format.revlog-compression=zlib

(This is disabling some recent hg features which make vc-hg-state use
the slow path.  You can verify that vc-hg-state-fast is going to run by
checking that .hg/requirements is a subset of
vc-hg-supported-requirements)

2. touch foo && hg add foo && hg commit -m.
3. (vc-hg-state-slow ".")
Returns 'up-to-date which is correct (because it shells out to hg)
4. (vc-hg-state ".")
Returns 'unregistered which is wrong

To a user, this manifests as various hg commands failing with errors,
often something like
"vc-find-backend-function: Cannot open load file: No such file or directory, vc-nil"

This also happens on Emacs 29.

(Note that this is independent of my other bug about lack of support for
certain repo requirements.  That bug is about expanding vc-hg-state-fast
support to new repos; this bug is showing that the already existing
support is buggy!)


In GNU Emacs 28.2 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo version 1.15.12, Xaw scroll bars)
 of 2023-02-09 built on igm-qws-u12685a
Repository revision: 739b5d0e52d83ec567bd61a5a49ac0e93e0eb469
Repository branch: HEAD
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: CentOS Linux 7 (Core)

Configured using:
 'configure --with-x-toolkit=lucid --without-gpm --without-gconf
 --without-selinux --without-imagemagick --with-modules --with-gif=no
 --with-cairo --with-rsvg
 --prefix=/j/office/app/emacs/builds/28.2-20230209-135825'

Configured features:
CAIRO DBUS FREETYPE GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON LIBXML2
MODULES NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND THREADS TIFF
TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM LUCID ZLIB

Important settings:
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Major mode: Dired by name

Minor modes in effect:
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Load-path shadows:
None found.

Features:
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mule custom widget hashtable-print-readable backquote threads dbusbind
inotify dynamic-setting system-font-setting font-render-setting cairo
x-toolkit x multi-tty make-network-process emacs)

Memory information:
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