Do nothing if converting to the same type of Note

The advantage is that the buffer will no longer be
marked as dirty when converting to the same type.

A quick break here is done instead of disabling the
LFUN because disabling the LFUN would lead to a greyed out
entry, which might confuse users. In the future, we might
want to have a radio button for switching between notes.
Merge remote-tracking branch 'github/build-bot-master' into bleeding-edge-master
2014.11.18-01.00