ref-filter: rename 'FILTER_REFS_ALL' to 'FILTER_REFS_REGULAR'

ref-filter: rename 'FILTER_REFS_ALL' to 'FILTER_REFS_REGULAR'

The flag 'FILTER_REFS_ALL' is a bit ambiguous, where ALL doesn't specify
if it means to contain refs from all worktrees or whether all types of
refs (regular, HEAD & pseudorefs) or all of the above.

Since here it is actually referring to all refs with the "refs/" prefix,
let's rename it to 'FILTER_REFS_REGULAR' to indicate that this is
specifically for regular refs.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>