has_sha1_pack(): refactor "pretend these packs do not exist" interface

has_sha1_pack(): refactor "pretend these packs do not exist" interface

Most of the callers of this function except only one pass NULL to its last
parameter, ignore_packed.

Introduce has_sha1_kept_pack() function that has the function signature
and the semantics of this function, and convert the sole caller that does
not pass NULL to call this new function.

All other callers and has_sha1_pack() lose the ignore_packed parameter.

Signed-off-by: Junio C Hamano <gitster@pobox.com>