pack-objects: validation and documentation about unreachable options
These options are added in [1] [2] [3]. All these depend on running
rev-list internally which is normally true since they are always used
with "--all --objects" which implies --revs. But let's keep this
dependency explicit.
While at there, add documentation for them. These are mostly used
internally by git-repack. But it's still good to not chase down the
right commit message to know how they work.
[1]
ca11b212eb (let pack-objects do the writing of unreachable objects
as loose objects - 2008-05-14)
[2]
08cdfb1337 (pack-objects --keep-unreachable - 2007-09-16)
[3]
e26a8c4721 (repack: extend --keep-unreachable to loose objects -
2016-06-13)
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>