revision: move bloom keyvec precondition into function

revision: move bloom keyvec precondition into function

There are currently two callsites calling
check_maybe_different_in_bloom_filter(). They both check if
revs->bloom_keyvecs_nr is not zero before they call that function.

Move bloom_keyvecs_nr precondition into
check_maybe_different_in_bloom_filter() to simplify the code.

Note that this changes `bloom_ret` to become -1 when there are no Bloom
key vectors, which results in `count_bloom_filter_false_positive` not
being incremented. This is unobservable, as the Bloom statistics are
only reported when key vectors were set up.

Signed-off-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>