graph API: fix "git log --graph --first-parent"

graph API: fix "git log --graph --first-parent"

This change teaches the graph API that only the first parent of each
commit is interesting when "--first-parent" was specified.

This change also consolidates the graph parent walking logic into two
new internal functions, first_interesting_parent() and
next_interesting_parent().  A simpler fix would have been to simply
break at the end of the 2 existing for loops when
graph->revs->first_parent_only is set.  However, this change seems
nicer, especially if we ever need to add any new loops over the parent
list in the future.

Signed-off-by: Adam Simpkins <adam@adamsimpkins.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>