show-branch: color the commit status signs

show-branch: color the commit status signs

Make it possible to color the status character ('*' '!' '+' '-') of each
commit corresponding to the branch it's in. This makes it easier to
follow a particular branch, especially if there are larger gaps in the
output.

Add the config option color.showbranch and the command line options
 --color and --no-color to control the colored output.

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>