t/perf: implement performance tests for pseudo-merge bitmaps

Implement a straightforward performance test demonstrating the benefit
of pseudo-merge bitmaps by measuring how long it takes to count
reachable objects in a few different scenarios:

  - without bitmaps, to demonstrate a reasonable baseline
  - with bitmaps, but without pseudo-merges
  - with bitmaps and pseudo-merges

Results from running this test on git.git are as follows:

    Test                                                                this tree
    -----------------------------------------------------------------------------------
    5333.2: git rev-list --count --all --objects (no bitmaps)           3.54(3.45+0.08)
    5333.3: git rev-list --count --all --objects (no pseudo-merges)     0.43(0.40+0.03)
    5333.4: git rev-list --count --all --objects (with pseudo-merges)   0.12(0.11+0.01)

On a private repository which is much larger, and has many spikey parts
of history that aren't merged into the 'master' branch, the results are
as follows:

    Test                                                                this tree
    ---------------------------------------------------------------------------------------
    5333.1: git rev-list --count --all --objects (no bitmaps)           122.29(121.31+0.97)
    5333.2: git rev-list --count --all --objects (no pseudo-merges)     21.88(21.30+0.58)
    5333.3: git rev-list --count --all --objects (with pseudo-merges)   5.05(4.77+0.28)

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

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