for-each-ref: introduce color format

You can now do something like

$ git for-each-ref --format='%C(red)%(refname:short)%C(reset)
%C(blue)%(upstream:diff)%C(reset)' --count 5 --sort='-committerdate'
refs/heads

To get output that's much more customizable 'git branch' output.  Future
patches will attempt unify the semantics of 'git branch' and 'git
for-each-ref'.

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