warning: push.default is unset; its implicit value is changing in
Git 2.0 from 'matching' to 'simple'. To squelch this message
and maintain the current behavior after the default changes, use:
git config --global push.default matching
To squelch this message and adopt the new behavior now, use:
git config --global push.default simple
See 'git help config' and search for 'push.default' for further information.
(the 'simple' mode was introduced in Git 1.7.11. Use the similar mode
'current' instead of 'simple' if you sometimes use older versions of Git)
Git 2.0でpush.defaultのデフォルト値がmatchingからsimpleに変わった - Humanity
simple のほうが安全らしいので以下のようにして解決。
git config --global push.default simple