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>

strbuf.h

@@ -357,20 +357,6 @@ size_t strbuf_expand_literal_cb(struct strbuf *sb,
 				const char *placeholder,
 				void *context);
 
-/**
- * Used as callback for `strbuf_expand()`, expects an array of
- * struct strbuf_expand_dict_entry as context, i.e. pairs of
- * placeholder and replacement string.  The array needs to be
- * terminated by an entry with placeholder set to NULL.
- */
-struct strbuf_expand_dict_entry {
-	const char *placeholder;
-	const char *value;
-};
-size_t strbuf_expand_dict_cb(struct strbuf *sb,
-			     const char *placeholder,
-			     void *context);
-
 /**
  * If the string pointed to by `formatp` contains a percent sign ("%"),
  * advance it to point to the character following the next one and