Retain caches of submodule refs

Retain caches of submodule refs

Instead of keeping track of one cache for refs in the main repo and
another single cache shared among submodules, keep a linked list of
cached_refs objects, one for each module/submodule. Change
invalidate_cached_refs() to invalidate all caches. (Previously, it
only invalidated the cache of the main repo because the submodule
caches were not reused anyway.)

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>