bug#42966: 28.0.50; vc-dir: wrong backend

[Sam Steingold <sds@gnu.org>]

My home directory is under `git` and `~/.gitignore` contains `/src` so
that directories under `~/src` can be handled separately, under
different repos.

`~/src/cl/clocc` is a managed by mercurial and files there are identified
by Emacs as such, as indicated in the mode line as `Hg:1234567`.

However, when I do `C-x v d` in a buffer visiting
`~/src/cl/clocc/src/port/proc.lisp`, I get a prompt

VC status for directory: ~/src/cl/clocc/

(note the _correct_ default mercurial root directory!) and hit RET, I
get the `*vc-dr*` buffer with

--8<---------------cut here---------------start------------->8---
VC backend : Git
Working dir: ~/src/cl/clocc/
Branch     : master
Remote     : git@gitlab.com:sam-s/home.git
Stash      : Nothing stashed

                         ./

--8<---------------cut here---------------end--------------->8---

note the incorrect `VC backend` and `Remote` (which should be `Hg` and
`ssh://sds@hg.code.sf.net/p/clocc/hg` resp.)

This is because `vc-responsible-backend` calls `responsible-p` and
`vc-git-responsible-p` is aliased to `vc-git-root` which merely looks
for `.git` above the argument, not paying attention to `.gitignore`.




In GNU Emacs 28.0.50 (build 6, x86_64-apple-darwin19.6.0, NS appkit-1894.60 Version 10.15.6 (Build 19G2021))
 of 2020-08-17 built on BZ-C02XR5CGJG5L
Repository revision: e5d4fae6797330d91e901c7ecb1412551db12f6a
Repository branch: master
Windowing system distributor 'Apple', version 10.3.1894
System Description:  Mac OS X 10.15.6
Configured using:
 'configure --with-imagemagick --with-mailutils --with-ns
 PKG_CONFIG_PATH=/usr/local/opt/libxml2/lib/pkgconfig:/usr/local/opt/imagemagick/lib/pkgconfig:/usr/local/opt/gnutls/lib/pkgconfig:/usr/local/opt/jansson/lib/pkgconfig:/usr/local/opt/libtiff/lib/pkgconfig:/usr/local/opt/libpng/lib/pkgconfig:/usr/local/opt/libjpeg/lib/pkgconfig:/usr/local/opt/freetype/lib/pkgconfig'

Configured features:
JPEG TIFF PNG IMAGEMAGICK NOTIFY KQUEUE ACL GNUTLS LIBXML2 ZLIB
TOOLKIT_SCROLL_BARS NS MODULES THREADS JSON PDUMPER LCMS2

Important settings:
  value of $LANG: C
  locale-coding-system: utf-8-unix

Features:
(shadow bbdb-message mailalias cookie1 emacsbug sendmail cl-indent
face-remap inf-lisp vc-hg url-http url-gw url-auth url-queue url-cache
gnus-fun time nndoc flow-fill tramp-cmds sort gnus-cite smiley
mm-archive gnus-async gnus-bcklg gnus-dup qp mail-extr gnus-ml hl-line
disp-table spam spam-stat gnus-uu yenc nndraft nnmh gnus-agent gnus-srvr
gnus-score score-mode nnvirtual gnus-msg gnus-art mm-uu mml2015 mm-view
mml-smime smime dig utf-7 gnus-cache gnus-sum url url-proxy url-privacy
url-expand url-methods url-history mailcap shr url-cookie url-domsuf
url-util svg xml dom bbdb-gnus gnutls network-stream nsm nntp gnus-group
gnus-undo gnus-start gnus-cloud nnimap nnmail mail-source utf7 netrc
nnoo gnus-spec gnus-int gnus-range gnus-win doc-view jka-compr
image-mode exif vc-annotate pulse tabify cl-print debug backtrace
tramp-cache cal-move cal-x view cal-china cal-bahai cal-islam holidays
hol-loaddefs bbdb-anniv cal-iso cal-hebrew lunar cal-julian solar
cal-dst appt diary-lib diary-loaddefs cal-menu calendar cal-loaddefs
find-dired bug-reference eieio-opt speedbar ezimage dframe find-func
misearch multi-isearch log-view skeleton dabbrev log-edit message rmc
puny rfc822 mml mml-sec epa derived epg epg-config mm-decode mm-bodies
mm-encode mail-parse rfc2231 gmm-utils mailheader pcvs-util smerge-mode
diff vc-bzr vc-src vc-sccs vc-svn vc-cvs vc-rcs add-log remember
tex-mode dired-aux dired dired-loaddefs inf-ruby ruby-mode yaml-mode
vc-dir ewoc vc vc-dispatcher sh-script smie executable conf-mode
company-oddmuse company-keywords company-etags company-gtags
company-dabbrev-code company-dabbrev company-files company-clang
company-template company-cmake company-bbdb yasnippet-snippets cl-extra
yasnippet flymake-proc flymake thingatpt company-capf company pcase
help-fns radix-tree help-mode elpy edmacro kmacro elpy-rpc pyvenv eshell
esh-cmd esh-ext esh-opt esh-proc esh-io esh-arg esh-module esh-groups
esh-util elpy-shell elpy-profile elpy-django s elpy-refactor ido grep
compile etags fileloop generator xref project cus-edit python tramp-sh
tramp tramp-loaddefs trampver tramp-integration files-x tramp-compat
shell pcomplete parse-time iso8601 ls-lisp format-spec comint ansi-color
vc-git diff-mode easy-mmode flyspell ispell midnight warnings gnus
nnheader gnus-util rmail rmail-loaddefs rfc2047 rfc2045 ietf-drums
text-property-search time-date mail-utils mm-util mail-prsvr wid-edit
bbdb-mua bbdb-com crm mailabbrev bbdb bbdb-site timezone edit-server
advice server winner ring which-func imenu paren help-at-pt desktop
frameset cus-start cus-load info package easymenu browse-url
url-handlers url-parse auth-source cl-seq eieio eieio-core cl-macs
eieio-loaddefs password-cache json subr-x map url-vars seq byte-opt gv
bytecomp byte-compile cconv cl-loaddefs cl-lib tooltip eldoc electric
uniquify ediff-hook vc-hooks lisp-float-type mwheel term/ns-win ns-win
ucs-normalize mule-util term/common-win tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode elisp-mode
lisp-mode prog-mode register page tab-bar menu-bar rfn-eshadow isearch
timer select scroll-bar mouse jit-lock font-lock syntax facemenu
font-core term/tty-colors frame minibuffer cl-generic cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray
cl-preloaded nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote threads kqueue cocoa ns
lcms2 multi-tty make-network-process emacs)

Memory information:
((conses 16 679755 110407)
 (symbols 48 32339 6)
 (strings 32 227857 7588)
 (string-bytes 1 6102300)
 (vectors 16 88150)
 (vector-slots 8 2148274 178424)
 (floats 8 1143 845)
 (intervals 56 46064 762)
 (buffers 992 121))

-- 
Sam Steingold (http://sds.podval.org/) on darwin Ns 10.3.1894
http://childpsy.net http://calmchildstories.com http://steingoldpsychology.com
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Daddy, why doesn't this magnet pick up this floppy disk?