New git 2.0 ready config option: push.default

Preping for git 2.0... gotta set push.default.
In my git workflow, I'm usually working on a branch named the same
branch I am pushing to. So instead of taking 'matching' or 'simple' I
took 'current' instead.

Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>