replace strbuf_expand_dict_cb() with strbuf_expand_step()

replace strbuf_expand_dict_cb() with strbuf_expand_step()

Avoid the overhead of setting up a dictionary and passing it via
strbuf_expand() to strbuf_expand_dict_cb() by using strbuf_expand_step()
in a loop instead.  It requires explicit handling of %% and unrecognized
placeholders, but is more direct and simpler overall, and expands only
on demand.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>