name-rev: Fix non-shortest description

name-rev: Fix non-shortest description

Uwe Kleine-König noticed that under certain circumstances, name-rev
picked a non-optimal tag.  Jeff King analyzed that name-rev only
takes into account the number of merge traversals, and then the
_last_ number in the description.

As an easy way to fix it, use a weighting factor for merge traversals:
A merge traversal is now made 65535 times more expensive than a
first-parent traversal.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Uwe Kleine-König <ukleinek@informatik.uni-freiburg.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>