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 <email@example.com> Signed-off-by: Junio C Hamano <firstname.lastname@example.org>