Implement wrap format %w() as if it is a mode switch

Implement wrap format %w() as if it is a mode switch

I always considered line wrapping to be more similar to a colour, i.e. a
state that one can change and that is applied to all following text until
the next state change, except that it's always reset at the end of the
format string.

Here's a patch to implement this behaviour, using Dscho's
strbuf_add_wrapped_text()

Signed-off-by: Junio C Hamano <gitster@pobox.com>