show-branch: make the current branch and merge commits stand out.

show-branch: make the current branch and merge commits stand out.

This changes the character used to mark the commits that is on the
branch from '+' to '*' for the current branch, to make it stand out.
Also we show '-' for merge commits.

When you have a handful branches with relatively long diversion, it
is easier to see which one is the current branch this way.

Signed-off-by: Junio C Hamano <junkio@cox.net>