Re: vc-git-previous-revision and cmd.exe

[Eli Zaretskii <eliz@gnu.org>]

> From: Andreas Schwab <schwab@linux-m68k.org>
> Cc: emacs-devel@gnu.org
> Date: Mon, 17 Nov 2014 18:55:10 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > vc-git-previous-revision appends "^" to a revision to get the previous
> > one.  But "^" is a special character for the Windows cmd.exe shell, so
> > 'D' in the *vc-change-log* buffer doesn't work if the 'git' command is
> > actually a batch file that invokes the real Git executable, in which
> > case we call cmd.exe to run the batch file, and cmd.exe removes the ^
> > (because it's the escape character).
> 
> Why is that a problem?  process-file should properly quote the arguments
> when it needs to go through a shell.

It's a Windows-specific conundrum: '^' is only special to cmd.exe, not
to argv[] processing of any other .exe program.  On top of that, the
only way to escape '^' is to double it.  But if we always double it,
then passing it to anything except cmd.exe will result in 2 ^^ in the
program's argv[].  And since most of the programs we invoke are not
cmd.exe or batch files, we don't quote '^' in shell-quote-argument.

On top of that, I don't think process-file knows it will go through a
shell in this case, since git.cmd it finds is just an executable file
for it.

(I bumped into this because 'D' in the *vc-change-log* buffer claimed
there were "no differences between 1234567 and 123456789...abc",
because cmd.exe removed the ^ part.)