Add threaded versions of functions in symlinks.c.

Add threaded versions of functions in symlinks.c.

check_leading_path() and has_dirs_only_path() both always use the default
cache, which could be a caveat for adding parallelism (which is a concern
and even a GSoC proposal).

Reimplement these two in terms of new threaded_check_leading_path() and
threaded_has_dirs_only_path() that take their own copy of the cache.

Signed-off-by: Jared Hance <jaredhance@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>