utf8.c: remove print_spaces()

utf8.c: remove print_spaces()

The previous patch made sure that strbuf_add_wrapped_text() (and thus
strbuf_add_indented_text(), too) always get a strbuf.  Make use of
this fact by adding strbuf_addchars(), a small helper that adds a
char the specified number of times to a strbuf, and use it to replace
print_spaces().

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>