Give the hunk comment its own color

Give the hunk comment its own color

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: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>