graph API: improve display of merge commits

graph API: improve display of merge commits

This change improves the way merge commits are displayed, to eliminate a
few visual artifacts.  Previously, merge commits were displayed as:

| M  \
| |\  |

As pointed out by Teemu Likonen, this didn't look nice if the rightmost
branch line was displayed as '\' on the previous line, as it then
appeared to have an extra space in it:

| |\
| M  \
| |\  |

This change updates the code so that branch lines to the right of merge
commits are printed slightly differently depending on how the previous
line was displayed:

| |\          | | |        | |  /
| M \         | M |        | M |
| |\ \        | |\ \       | |\ \

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