revision: make helper for pathspec to bloom keyvec

revision: make helper for pathspec to bloom keyvec

When preparing to use bloom filters in a revision walk, Git populates a
boom_keyvec with an array of bloom keys for the components of a path.
Before we create the ability to map multiple pathspecs to multiple
bloom_keyvecs, extract the conversion from a pathspec to a bloom_keyvec
into its own helper method. This simplifies the state that persists in
prepare_to_use_bloom_filter() as well as makes the future change much
simpler.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Lidong Yan <502024330056@smail.nju.edu.cn>
Signed-off-by: Junio C Hamano <gitster@pobox.com>