strbuf: separate callback for strbuf_expand:ing literals

Expanding '%n' and '%xNN' is generic functionality, so extract that from
the pretty.c formatter into a callback that can be reused.

No functional change intended

Signed-off-by: Anders Waldenborg <anders@0x63.nu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.h

@@ -320,6 +320,14 @@ void strbuf_expand(struct strbuf *sb,
 		   expand_fn_t fn,
 		   void *context);
 
+/**
+ * Used as callback for `strbuf_expand` to only expand literals
+ * (i.e. %n and %xNN). The context argument is ignored.
+ */
+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