Git: Fix thinko when trying to find merged branches

Fix thinko when trying to find out whether a branch was merged or not.

Change-Id: I6c4d600508af8a68fe0bac7e61f0b912c43cec32
Reviewed-on: http://codereview.qt.nokia.com/2200


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Robert Löhning <robert.loehning@nokia.com>