bug#39380: 26.3: Opening files in vc-dir-mode with differing root and working dir fails

[Eli Zaretskii]

> Cc: Eli Zaretskii <eliz@gnu.org>
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Thu, 6 Feb 2020 16:38:54 +0300
> 
> >> Good catch, thank you. Could you try this patch?
> >>
> > Works fine. Directory listing in vc-dir-mode is truncated at sub-directory. 
> > Files are processed correctly.
> 
> Very good.
> 
> Eli, I'd like to get it into emacs-27. OK?

I'll have to ask you to walk me through the change and explain how it
solves the original issue.

I tried to figure that out myself, but it's too hard: the patch has no
log message, the function being changed has no doc string which could
describe its contract, and the Mercurial documentation I've read seems
to contradict what you are doing (e.g., it says "re:" should be
immediately followed by a regexp).  Sorry, I cannot make up my mind
whether this is a safe change without understanding it better.

Thanks.