add strbuf_expand_dict_cb(), a helper for simple cases

add strbuf_expand_dict_cb(), a helper for simple cases

The new callback function strbuf_expand_dict_cb() can be used together
with strbuf_expand() if there is only a small number of placeholders
for static replacement texts.  It expects its dictionary as an array of
placeholder+value pairs as context parameter, terminated by an entry
with the placeholder member set to NULL.

The new helper is intended to aid converting the remaining calls of
interpolate().  strbuf_expand() is smaller, more flexible and can be
used to go faster than interpolate(), so it should replace the latter.

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