From git v1.7.11 release notes:
A new mode for push, "simple", which is a cross between "current"
and "upstream", has been introduced. "git push" without any refspec
will push the current branch out to the same name at the remote
repository only when it is set to track the branch with the same
name over there. The plan is to make this mode the new default
value when push.default is not configured.
Use new mode:
git config --global push.default simple