Inspired by the coloring of quilt.

Introduce a separate color and paint the hunk comment part, i.e. the name
of the function, in a separate color "diff.func" (defaults to plain).

Whitespace between hunk header and hunk comment is printed in plain color.
Signed-off-by: NBert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>