Re: [2.22.0] difftool no longer passes through to git diff if diff.tool is unset

[Denton Liu <liu.denton@gmail.com>]

On Wed, Jun 19, 2019 at 09:17:44PM -0400, Denton Liu wrote:
> Hi Logan,
> 
> On Wed, Jun 19, 2019 at 11:54:22PM +0000, Pugh, Logan wrote:
> > Note: This issue was originally discussed on this StackOverflow thread: https://stackoverflow.com/q/56675863
> > 
> > Prior to Git version 2.22.0 I was able to use git difftool without configuring diff.tool or merge.tool and it would show the diff using git diff.
> > 
> > E.g. with Git 2.21.0:
> > 
> > ~/gits/src/git$ git difftool --no-index color.c color.h
> > diff --git a/color.c b/color.h
> > index ebb222ec33..98894d6a17 100644
> > 
> > In Git version 2.22.0 an error message about diff.tool not being configured is displayed:
> > 
> > ~/gits/src/git$ git difftool --no-index color.c color.h
> > 
> > This message is displayed because 'diff.tool' is not configured.
> > See 'git difftool --tool-help' or 'git help config' for more details.
> > 'git difftool' will now attempt to use one of the following tools:
> > kompare emerge vimdiff
> > 
> > The following commit is thought to have caused the regression: https://github.com/git/git/commit/05fb8726cccc74908853c166248ff9b6abdafae5
> > 
> > Please let me know if I can provide any more info.
> > 
> > Thanks,
> > 
> > Logan
> 
> Thanks for the report. I'll take a look at this later today.

Using the following command on git.git,

	$ HOME=/dev/null ./git --exec-path=. difftool --no-index color.c color.h

I did a quick bisect on the issue and it seems like the cause of this
bug is actually 287ab28bfa (diff: reuse diff setup for --no-index case,
2019-02-16). I'll do a bit more digging tomorrow.

(CCing Peff into this)