.gitconfig: set push.default simple

This is the planned new default setting and will stop git from nagging
me when pushing. From Git v1.8.0 release notes:

Backward compatibility notes
----------------------------

In the next major release (not *this* one), we will change the
behavior of the "git push" command.

When "git push [$there]" does not say what to push, we have used the
traditional "matching" semantics so far (all your branches were sent
to the remote as long as there already are branches of the same name
over there).  We will use the "simple" semantics that pushes the
current branch to the branch with the same name, only when the current
branch is set to integrate with that remote branch.  There is a user
preference configuration variable "push.default" to change this, and
"git push" will warn about the upcoming change until you set this
variable in this release.

"git branch --set-upstream" is deprecated and may be removed in a
relatively distant future.  "git branch [-u|--set-upstream-to]" has
been introduced with a saner order of arguments.