hashmap: convert sha1hash() to oidhash()

There are no callers left of sha1hash() that do not simply pass the
"hash" member of a "struct object_id". Let's get rid of the outdated
sha1-specific function and provide one that operates on the whole struct
(even though the technique, taking the first few bytes of the hash, will
remain the same).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

object.c

@@ -61,7 +61,7 @@ int type_from_string_gently(const char *str, ssize_t len, int gentle)
  */
 static unsigned int hash_obj(const struct object_id *oid, unsigned int n)
 {
-	return sha1hash(oid->hash) & (n - 1);
+	return oidhash(oid) & (n - 1);
 }
 
 /*