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)

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
for-each-ref: auto reset color after each atom

It makes it easier to write a sensible format string, since you don't
have to %(color:reset) after each atom. Additionally, to make sure that
an invocation like the following doesn't leak color,

  $ git for-each-ref --format='%(subject)%(color:green)'

auto-reset at the end of the format string as well.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>