graph API: don't print branch lines for uninteresting merge parents
Previously, the graphing code printed lines coming out of a merge commit
for all of its parents, even if some of them were uninteresting. Now it
only prints lines for interesting commits.
For example, for a merge commit where only the first parent is
interesting, the code now prints:
* merge commit
* interesting child
instead of:
M merge commit
|\
* interesting child
Signed-off-by: Adam Simpkins <adam@adamsimpkins.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>