Fix the HEAD updating during a conflicting push (bug #12609)

Fix the HEAD updating during a conflicting push (bug #12609)

Copying most of Karl's comment from the list:

 1. In push_patch(), we delay the final stack update (the update()
    function) since we want to record the state just before the
    conflict in the stack log.

 2. In run(), we update the branch head before running the delayed
    stack update (self.__conflicting_push()).

The patch works around this problem by explicitly specifying what the
branch head should be; this mechanism is used by undo etc. to be able
to set the branch head to something that isn't the stack top.

Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>
Acked-by: Karl Hasselström <kha@treskal.com>