for_each_*_object: take flag arguments as enum

It's not wrong to pass our flags in an "unsigned", as we
know it will be at least as large as the enum.  However,
using the enum in the declaration makes it more obvious
where to find the list of flags.

While we're here, let's also drop the "extern" noise-words
from the declarations, per our modern coding style.

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

cache.h

@@ -1640,7 +1640,8 @@ enum for_each_object_flags {
  * repository and any alternates repositories (unless the
  * LOCAL_ONLY flag is set).
  */
-extern int for_each_loose_object(each_loose_object_fn, void *, unsigned flags);
+int for_each_loose_object(each_loose_object_fn, void *,
+			  enum for_each_object_flags flags);
 
 /*
  * Set this to 0 to prevent sha1_object_info_extended() from fetching missing