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

["Pugh, Logan" <Logan.Pugh@austintexas.gov>]

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