git makes it impossible to ignore or override global gitconfig, needs `GITCONFIGFILE`

git makes it impossible to ignore global gitconfig. This is essentially what's being asked here: https://stackoverflow.com/questions/23400449/how-to-make-git-temporarily-ignore-gitconfig

## proposal: add an environment variable (like for tig and other programs), eg: `GITCONFIGFILE=~/.gitconfig_temp git diff` seems like an easy change that would make it easy for users to ignore or override their gitconfig

## note: I've asked here: https://groups.google.com/forum/#!topic/git-users/1ehWsItL8vE but got 0 response after 6 months