object-name: move related functions to object-name

Move object-name-related functions from strbuf.[ch] to object-name.[ch]
so that strbuf is focused on string manipulation routines with minimal
dependencies.

dir.h relied on the forward declration of the repository struct in
strbuf.h. Since that is removed in this patch, add the forward
declaration to dir.h.

Signed-off-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

dir.h

@@ -40,6 +40,8 @@
  *
  */
 
+struct repository;
+
 struct dir_entry {
 	unsigned int len;
 	char name[FLEX_ARRAY]; /* more */