graph API: avoid printing unnecessary padding before some octopus merges

graph API: avoid printing unnecessary padding before some octopus merges

When an octopus merge is printed, several lines are printed before it to
move over existing branch lines to its right.  This is needed to make
room for the children of the octopus merge.  For example:

| | | |
| |  \ \
| |   \ \
| |    \ \
| M---. \ \
| |\ \ \ \ \

However, this step isn't necessary if there are no branch lines to the
right of the octopus merge.  Therefore, skip this step when it is not
needed, to avoid printing extra lines that don't really serve any
purpose.

Signed-off-by: Adam Simpkins <adam@adamsimpkins.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>