The purpose of --first-parent is to view the tree without looking at
side branche. This is accomplished by pretending there are no other
parents than the first parent when encountering a merge.
The current code marks the other parents as seen, which means that the tree
traversal will behave differently depending on the order merges are handled.
When a fast forward is artificially recorded as a merge,
-----
/ \
D---E---F---G master
the current first-parent code considers E to be seen and stops the
traversal after showing G and F.
Signed-off-by: Stephen R. van den Berg <srb@cuci.nl> Signed-off-by: Junio C Hamano <gitster@pobox.com>