builtin/repack: fix `--keep-unreachable` when there are no packs

The "--keep-unreachable" flag is supposed to append any unreachable
objects to the newly written pack. This flag is explicitly documented as
appending both packed and loose unreachable objects to the new packfile.
And while this works alright when repacking with preexisting packfiles,
it stops working when the repository does not have any packfiles at all.

The root cause are the conditions used to decide whether or not we want
to append "--pack-loose-unreachable" to git-pack-objects(1). There are
a couple of conditions here:

- `has_existing_non_kept_packs()` checks whether there are existing
packfiles. This condition makes sense to guard "--keep-pack=",
"--unpack-unreachable" and "--keep-unreachable", because all of
these flags only make sense in combination with existing packfiles.
But it does not make sense to disable `--pack-loose-unreachable`
when there aren't any preexisting packfiles, as loose objects can be
packed into the new packfile regardless of that.

- `delete_redundant` checks whether we want to delete any objects or
packs that are about to become redundant. The documentation of
`--keep-unreachable` explicitly says that `git repack -ad` needs to
be executed for the flag to have an effect.

It is not immediately obvious why such redundant objects need to be
deleted in order for "--pack-unreachable-objects" to be effective.
But as things are working as documented this is nothing we'll change
for now.

- `pack_everything & PACK_CRUFT` checks that we're not creating a
cruft pack. This condition makes sense in the context of
"--pack-loose-unreachable", as unreachable objects would end up in
the cruft pack anyway.

So while the second and third condition are sensible, it does not make
any sense to condition `--pack-loose-unreachable` on the existence of
packfiles.

Fix the bug by splitting out the "--pack-loose-unreachable" and only
making it depend on the second and third condition. Like this, loose
unreachable objects will be packed regardless of any preexisting
packfiles.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>