object.h: move some inline functions and defines from cache.h

object.h: move some inline functions and defines from cache.h

The object_type() inline function is very tied to the enum object_type
declaration within object.h, and just seemed to make more sense to live
there.  That makes S_ISGITLINK and some other defines make sense to go
with it, as well as the create_ce_mode() and canon_mode() inline
functions.  Move all these inline functions and defines from cache.h to
object.h.

Signed-off-by: Elijah Newren <newren@gmail.com>
Acked-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>