midx: avoid duplicate packed_git entries

When we scan a pack directory to load the idx entries we find into the
packed_git list, we skip any of them that are contained in a midx. We
then load them later lazily if we actually need to access the
corresponding pack, referencing them both from the midx struct and the
packed_git list.

The lazy-load in the midx code checks to see if the midx already
mentions the pack, but doesn't otherwise check the packed_git list. This
makes sense, since we should have added any pack to both lists.

But there's a loophole! If we call close_object_store(), that frees the
midx entirely, but _not_ the packed_git structs, which we must keep
around for Reasons[1]. If we then try to look up more objects, we'll
auto-load the midx again, which won't realize that we've already loaded
those packs, and will create duplicate entries in the packed_git list.

This is possibly inefficient, because it means we may open and map the
pack redundantly. But it can also lead to weird user-visible behavior.
The case I found is in "git repack", which closes and reopens the midx
after repacking and then calls update_server_info(). We end up writing
the duplicate entries into objects/info/packs.

We could obviously de-dup them while writing that file, but it seems
like a violation of more core assumptions that we end up with these
duplicate structs at all.

We can avoid the duplicates reasonably efficiently by checking their
names in the pack_map hash. This annoyingly does require a little more
than a straight hash lookup due to the naming conventions, but it should
only happen when we are about to actually open a pack. I don't think one
extra malloc will be noticeable there.

[1] I'm not entirely sure of all the details, except that we generally
    assume the packed_git structs never go away. We noted this
    restriction in the comment added by 6f1e9394e2 (object: fix leaking
    packfiles when closing object store, 2024-08-08), but it's somewhat
    vague. At any rate, if you try freeing the structs in
    close_object_store(), you can observe segfaults all over the test
    suite. So it might be fixable, but it's not trivial.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

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