builtin/repack.c: only collect fully-formed packs

To partition the set of packs based on which ones are "kept" (either
they have a .keep file, or were otherwise marked via the `--keep-pack`
option) and "non-kept" ones (anything else), `git repack` uses its
`collect_pack_filenames()` function.

Ordinarily, we would rely on a convenience function such as
`get_all_packs()` to enumerate and partition the set of packs. But
`collect_pack_filenames()` uses `readdir()` directly to read the
contents of the "$GIT_DIR/objects/pack" directory, and adds each entry
ending in ".pack" to the appropriate list (either kept, or non-kept as
above).

This is subtly racy, since `collect_pack_filenames()` may see a pack
that is not fully staged (i.e., it is missing its ".idx" file).
Ordinarily, this doesn't cause a problem. But it can cause issues when
generating a cruft pack.

This is because `git repack` feeds (among other things) the list of
existing kept packs down to `git pack-objects --cruft` to indicate that
any kept packs will not be removed from the repository (so that the
cruft pack machinery can avoid packing objects that appear in those
packs as cruft).

But `read_cruft_objects()` lists packfiles by calling `get_all_packs()`.
So if a ".pack" file exists (necessary to get that pack to appear to
`collect_pack_filenames()`), but doesn't have a corresponding ".idx"
file (necessary to get that pack to appear via `get_all_packs()`), we'll
complain with:

    fatal: could not find pack '.tmp-5841-pack-a6b0150558609c323c496ced21de6f4b66589260.pack'

Fix the above by teaching `collect_pack_filenames()` to only collect
packs with their corresponding `*.idx` files in place, indicating that
those packs have been fully staged.

There are a couple of things worth noting:

  - Since each entry in the `extra_keep` list (which contains the
    `--keep-pack` names) has a `*.pack` suffix, we'll have to swap the
    suffix from ".pack" to ".idx", and compare that instead.

  - Since we use the the `fname_kept_list` to figure out which packs to
    delete (with `git repack -d`), we would have previously deleted a
    `*.pack` with no index (since the existince of a ".pack" file is
    necessary and sufficient to include that pack in the list of
    existing non-kept packs).

    Now we will leave it alone (since that pack won't appear in the
    list). This is far more correct behavior, since we don't want
    to race with a pack being staged. Deleting a partially staged pack
    is unlikely, however, since the window of time between staging a
    pack and moving its .idx file into place is miniscule.

    Note that this window does *not* include the time it takes to
    receive and index the pack, since the incoming data goes into
    "$GIT_DIR/objects/tmp_pack_XXXXXX", which does not end in ".pack"
    and is thus ignored by collect_pack_filenames().

In the future, this function should probably be rewritten as a callback
to `for_each_file_in_pack_dir()`, but this is the simplest change we
could do in the short-term.

Reported-by: Michael Haggerty <mhagger@github.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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