list-objects: drop --unpacked non-commit objects from results

list-objects: drop --unpacked non-commit objects from results

In git-rev-list(1), we describe the `--unpacked` option as:

    Only useful with `--objects`; print the object IDs that are not in
    packs.

This is true of commits, which we discard via get_commit_action(), but
not of the objects they reach. So if we ask for an --objects traversal
with --unpacked, we may get arbitrarily many objects which are indeed
packed.

I am nearly certain this behavior dates back to the introduction of
`--unpacked` via 12d2a18780 ("git rev-list --unpacked" shows only
unpacked commits, 2005-07-03), but I couldn't get that revision of Git
to compile for me. At least as early as v2.0.0 this has been subtly
broken:

    $ git.compile --version
    git version 2.0.0

    $ git.compile rev-list --objects --all --unpacked
    72791fe96c93f9ec5c311b8bc966ab349b3b5bbe
    05713d991c18bbeef7e154f99660005311b5004d v1.0
    153ed8b7719c6f5a68ce7ffc43133e95a6ac0fdb
    8e4020bb5a8d8c873b25de15933e75cc0fc275df one
    9200b628cf9dc883a85a7abc8d6e6730baee589c two
    3e6b46e1b7e3b91acce99f6a823104c28aae0b58 unpacked.t

There, only the first, third, and sixth entries are loose, with the
remaining set of objects belonging to at least one pack.

The implications for this are relatively benign: bare 'git repack'
invocations which invoke pack-objects with --unpacked are impacted, and
at worst we'll store a few extra objects that should have been excluded.

Arguably changing this behavior is a backwards-incompatible change,
since it alters the set of objects emitted from rev-list queries with
`--objects` and `--unpacked`. But I argue that this change is still
sensible, since the existing implementation deviates from
clearly-written documentation.

The fix here is straightforward: avoid showing any non-commit objects
which are contained in packs by discarding them within list-objects.c,
before they are shown to the user. Note that similar treatment for
`list-objects.c::show_commit()` is not needed, since that case is
already handled by `revision.c::get_commit_action()`.

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>