graph API: don't print branch lines for uninteresting merge parents

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>