multi-pack-index: prepare 'repack' subcommand

In an environment where the multi-pack-index is useful, it is due
to many pack-files and an inability to repack the object store
into a single pack-file. However, it is likely that many of these
pack-files are rather small, and could be repacked into a slightly
larger pack-file without too much effort. It may also be important
to ensure the object store is highly available and the repack
operation does not interrupt concurrent git commands.

Introduce a 'repack' subcommand to 'git multi-pack-index' that
takes a '--batch-size' option. The subcommand will inspect the
multi-pack-index for referenced pack-files whose size is smaller
than the batch size, until collecting a list of pack-files whose
sizes sum to larger than the batch size. Then, a new pack-file
will be created containing the objects from those pack-files that
are referenced by the multi-pack-index. The resulting pack is
likely to actually be smaller than the batch size due to
compression and the fact that there may be objects in the pack-
files that have duplicate copies in other pack-files.

The current change introduces the command-line arguments, and we
add a test that ensures we parse these options properly. Since
we specify a small batch size, we will guarantee that future
implementations do not change the list of pack-files.

In addition, we hard-code the modified times of the packs in
the pack directory to ensure the list of packs sorted by modified
time matches the order if sorted by size (ascending). This will
be important in a future test.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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