for-each-ref: introduce %(color:...) for color

Enhance 'git for-each-ref' with color formatting options.  You can now
use the following format in for-each-ref:

%(color:green)%(refname:short)%(color:reset)

where color names are described in color.branch.*.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>