pack-objects: refactor `read_packs_list_from_stdin()` to use `strmap`

The '--stdin-packs' mode of pack-objects maintains two separate
string_lists: one for included packs, and one for excluded packs. Each
list stores the pack basename as a string and the corresponding
`packed_git` pointer in its `->util` field.

This works, but makes it awkward to extend the set of pack "kinds" that
pack-objects can accept via stdin, since each new kind would need its
own string_list and duplicated handling. A future commit will want to do
just this, so prepare for that change by handling the various "kinds" of
packs specified over stdin in a more generic fashion.

Namely, replace the two `string_list`s with a single `strmap` keyed on
the pack basename, with values pointing to a new `struct
stdin_pack_info`. This struct tracks both the `packed_git` pointer and a
`kind` bitfield indicating whether the pack was specified as included or
excluded.

Extract the logic for sorting packs by mtime and adding their objects
into a separate `stdin_packs_add_pack_entries()` helper.

While we could have used a `string_list`, we must handle the case where
the same pack is specified more than once. With a `string_list` only, we
would have to pay a quadratic cost to either (a) insert elements into
their sorted positions, or (b) a repeated linear search, which is
accidentally quadratic. For that reason, use a strmap instead.

This patch does not include any functional changes.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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