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>