Add documentation for git-branch's color configuration.

Add documentation for git-branch's color configuration.

Added color.branch and color.branch.<slot> to configuration list.
Style copied from color.status and meanings derived from the code.

Moved the color meanings from color.diff.<slot> to color.branch.<slot>
since the latter comes first alphabetically.

Added --color and --no-color to git-branch's usage and documentation.

Signed-off-by: Brian Gernhardt <benji@silverinsanity.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>