git 1.7.10 include directive and Local GitHub Config

The git 1.7.10 include directive [1] allows you to include external files from your .gitconfig. But these directives are not read by `git config --global github.user`, only by `git config github.user`.

In light of this, I think it makes sense for gist to use the latter rather than the former.

The same change was made to gh.el in sigma/gh.el#5.

https://github.com/gitster/git/commit/9b25a0b52e09400719366f0a33d0d0da98bbf7b0