Simplify is_kept_pack()

Simplify is_kept_pack()

This removes --unpacked=<packfile> parameter from the revision parser, and
rewrites its use in git-repack to pass a single --kept-pack-only option
instead.

The new --kept-pack-only option means just that.  When this option is
given, is_kept_pack() that used to say "not on the --unpacked=<packfile>
list" now says "the packfile has corresponding .keep file".

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