Prepare symlink caching for thread-safety

Prepare symlink caching for thread-safety

This doesn't actually change the external interfaces, so they are still
thread-unsafe, but it makes the code internally pass a pointer to a
local 'struct cache_def' around, so that the core code can be made
thread-safe.

The threaded index preloading will want to verify that the paths leading
up to a pathname are all real directories.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>