Add a library function to add an alternate to the alternates file

This is in the core so that, if the alternates file has already been
read, the addition can be parsed and put into effect for the current
process.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -599,6 +599,7 @@ extern struct alternate_object_database {
 	char base[FLEX_ARRAY]; /* more */
 } *alt_odb_list;
 extern void prepare_alt_odb(void);
+extern void add_to_alternates_file(const char *reference);
 
 struct pack_window {
 	struct pack_window *next;