Simplify and fix --first-parent implementation

Simplify and fix --first-parent implementation

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>