Git difftool does not work with `--check`

[lakshayg]

When I call :Git difftool --check, the quickfix list is populated but the entries don't jump to files. I would expect it to behave similar to:

:let old=&makeprg | set makeprg=git\ diff\ --check | make | let &makeprg=old

I suspect there's some issue with the 'errorformat' but I couldn't figure out how to set it in the plugin code.

[lakshayg]

Thanks for working on this! I found a small error when testing it locally and made a PR to fix it: #2305