stat_tracking_info(): only count real commits

stat_tracking_info(): only count real commits

stat_tracking_info() in remote.c is used to collect the statistics to
be able to say (for instance) from the output of "git checkout':

  Your branch and 'foo' have diverged,
  and have X and Y different commit(s) each, respectively.

Currently X and Y also includes the count of merges.  This patch
excludes the merges from being counted.

Signed-off-by: Kjetil Barvik <barvik@broadpark.no>
Signed-off-by: Junio C Hamano <gitster@pobox.com>