t/perf: add performance tests for multi-pack reuse

t/perf: add performance tests for multi-pack reuse

To ensure that we don't regress either the size or runtime performance
of multi-pack reuse, add a performance test to measure both of these.

The test partitions the objects in GIT_TEST_PERF_LARGE_REPO into 1, 10,
and 100 packs, and then tries to perform a "clone" at each stage with
both single- and multi-pack reuse enabled.

Note that the `repack_into_n_chunks()` function in this new test script
differs from the existing `repack_into_n()`. The former partitions the
repository into N equal-sized chunks, while the latter produces N packs
of five commits each (plus their objects), and then another pack with
the remainder.

On git.git, I can produce the following results on my machine:

    Test                                                            this tree
    --------------------------------------------------------------------------------
    5332.3: clone for 1-pack scenario (single-pack reuse)           1.57(2.99+0.15)
    5332.4: clone size for 1-pack scenario (single-pack reuse)               231.8M
    5332.5: clone for 1-pack scenario (multi-pack reuse)            1.79(2.96+0.21)
    5332.6: clone size for 1-pack scenario (multi-pack reuse)                231.7M
    5332.9: clone for 10-pack scenario (single-pack reuse)          3.89(16.75+0.35)
    5332.10: clone size for 10-pack scenario (single-pack reuse)             209.9M
    5332.11: clone for 10-pack scenario (multi-pack reuse)          1.56(2.99+0.17)
    5332.12: clone size for 10-pack scenario (multi-pack reuse)              224.4M
    5332.15: clone for 100-pack scenario (single-pack reuse)        8.24(54.31+0.59)
    5332.16: clone size for 100-pack scenario (single-pack reuse)            278.3M
    5332.17: clone for 100-pack scenario (multi-pack reuse)         2.13(2.44+0.33)
    5332.18: clone size for 100-pack scenario (multi-pack reuse)             357.9M

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