bug#60956: 27.2; project-try-vc considers every nonempty directory a root when vc-handled-backends is nil

[Jason Orendorff <jason.orendorff@gmail.com>]

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I observed that `eglot' would happily start dozens of language server
processes, but each one of them only seemed to know about a tiny
fraction of my code. I tracked this down and found a minor bug in
project.el.

In `project-try-vc', when `vc-handled-backends' is nil, the value of
`marker-re' computed here:

https://git.savannah.gnu.org/cgit/emacs.git/tree/lisp/progmodes/project.el?id=207901457c018d94b1ce9e13a897d8241b1f3af2#n516

is the empty string.

If no vc backends are enabled, no directory should be treated as a vc
root. But the empty regexp matches everything, so *all* directories are
treated as vc roots.

Consequently, `eglot' spawned a lot of processes, because it starts one
per project.

The workaround is to set `vc-handled-backends' to any other value. (I
suppose I configured it to nil long ago, when working in a huge
Mercurial repository. Back then, commonplace hg commands in that repo
were unreasonably slow.)

In GNU Emacs 27.2 (build 1, x86_64-apple-darwin18.7.0, NS appkit-1671.60
Version 10.14.6 (Build 18G95))
 of 2021-03-27 built on builder10-14.porkrind.org
Windowing system distributor 'Apple', version 10.3.2299
System Description:  macOS 13.0.1

Recent messages:
(".git")
project-try-vc-2
"~/src/blackbird/"
Quit
Followed link to /Users/jorendorff/src/dotfiles/.emacs
Mark set [3 times]
Quit
Making completion list...
Type "q" to restore previous buffer.
user-error: No cross-reference here
Quit
Configured using:
 'configure --with-ns '--enable-locallisppath=/Library/Application
 Support/Emacs/${version}/site-lisp:/Library/Application
 Support/Emacs/site-lisp' --with-modules'

Configured features:
NOTIFY KQUEUE ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES
THREADS JSON PDUMPER GMP

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Info

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Load-path shadows:
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/Users/jorendorff/.emacs.d/elpa/xref-1.6.0/xref hides
/Applications/Emacs.app/Contents/Resources/lisp/progmodes/xref
/Users/jorendorff/.emacs.d/elpa/project-0.9.3/project hides
/Applications/Emacs.app/Contents/Resources/lisp/progmodes/project
/Users/jorendorff/.emacs.d/elpa/flymake-1.2.2/flymake hides
/Applications/Emacs.app/Contents/Resources/lisp/progmodes/flymake
/Users/jorendorff/.emacs.d/elpa/seq-2.23/seq hides
/Applications/Emacs.app/Contents/Resources/lisp/emacs-lisp/seq
/Users/jorendorff/.emacs.d/elpa/eldoc-1.13.0/eldoc hides
/Applications/Emacs.app/Contents/Resources/lisp/emacs-lisp/eldoc

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Memory information:
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