wt-status: add helpers for printing wt-status lines

wt-status: add helpers for printing wt-status lines

Introduce status_printf{,_ln,_more} wrapper functions around
color_vfprintf() which take care of adding "#" to the beginning of
status lines automatically.  The semantics:

 - status_printf() is just like color_fprintf() but it adds a "# "
   at the beginning of each line of output;

 - status_printf_ln() is a convenience function that additionally
   adds "\n" at the end;

 - status_printf_more() is a variant of status_printf() used to
   continue lines that have already started.  It suppresses the "#" at
   the beginning of the first line.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>