bug#31497: 27.0.50; vc-hg doesn't separate standard error from standard output

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Wed, 09 Oct 2019 23:57:15 +0200
> Cc: 31497@debbugs.gnu.org
> 
> I can confirm that this bug is still present in Emacs 27.  The command
> is started from vc-doc-command, which uses start-file-process, which
> uses start-process, which doesn't even allow separating stderr from
> stdin.
> 
> (In general, Emacs stderr/stdin handling is very frustrating.)

>From the doc string of start-process:

  If you want to separate standard output from standard error, use
  ‘make-process’ or invoke the command through a shell and redirect
  one of them using the shell syntax.