enable user to set a upstream per repo as 'GUTTER_HEAD'

When GUTTER_HEAD is set in current repository, `git diff GUTTER_HEAD` will be used instead of `git diff`. 

It will be useful when (re)developing based on an upstream branch.

As an alternative, setting a branch name to `g:gitgutter_diff_args` will usually work, except when we need to develop with two or more repositories.