git log --graph: print '*' for all commits, including merges

git log --graph: print '*' for all commits, including merges

Previously, merge commits were printed with 'M' instead of '*'.  This
had the potential to confuse users when not all parents of the merge
commit were included in the log output.

As Junio has pointed out, merge commits can almost always be easily
identified from the log message, anyway.

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