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>