t/perf: add pack-objects filter and path-walk benchmark

Add p5315-pack-objects-filter.sh to measure the performance of
'git pack-objects --revs --all' under different filter and traversal
combinations:

 * no filter (baseline)
 * --filter=blob:none (blobless)
 * --filter=sparse:oid=<oid> (cone-mode sparse)

Each filter scenario is tested both with and without --path-walk,
producing paired measurements that show the impact of the path-walk
traversal for each filter type as we integrate the --path-walk feature
with different --filter options. It currently has no integration so
falls back to the standard revision walk. Thus, there are no significant
differences in the current results other than a full repack (and even
then, the --path-walk feature is not incredibly different for the
default Git repository):

Test                                             HEAD
-----------------------------------------------------
5315.2: repack (no filter)                      27.91
5315.3: repack size (no filter)                250.7M
5315.4: repack (no filter, --path-walk)         34.92
5315.5: repack size (no filter, --path-walk)   220.0M
5315.6: repack (blob:none)                      13.63
5315.7: repack size (blob:none)                137.6M
5315.8: repack (blob:none, --path-walk)         13.48
5315.9: repack size (blob:none, --path-walk)   137.7M
5315.10: repack (sparse:oid)                    72.67
5315.11: repack size (sparse:oid)              187.4M
5315.12: repack (sparse:oid, --path-walk)       72.47
5315.13: repack size (sparse:oid, --path-walk) 187.4M

The sparse filter definition is built automatically by sampling
depth-2 directories from the test repository, making the test work
on any repo passed via GIT_PERF_LARGE_REPO. For repos that lack
depth-2 directories, a single top-level directory is used; for flat
repos, the sparse tests are skipped via prerequisite.

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

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