MergeOp: Use Objects.equals when comparing branch tip

This currently works, because MergeTip doesn't copy or otherwise mess
around with the initial tip object passed in. But MergeTip doesn't
state that in its contract, so this may be a bug waiting to happen.

Change-Id: I61fd71c7f6895d4efb67edbec056b6534b8b94a4
1 file changed