t5326: test propagating hashcache values

Now that we both can propagate values from the hashcache, and respect the configuration to enable the hashcache at all, test that both of these function correctly by hardening their behavior with a test. Like the hash-cache in classic single-pack bitmaps, this helps more proportionally the more up-to-date your bitmap coverage is. When our bitmap coverage is out-of-date with the ref tips, we spend more time proportionally traversing, and all of that traversal gets the name-hash filled in. But for the up-to-date bitmaps, this helps quite a bit. These numbers are on git.git, with `pack.threads=1` to help see the difference reflected in the overall runtime. Test origin/tb/multi-pack-bitmaps HEAD ------------------------------------------------------------------------------------- 5326.4: simulated clone 1.87(1.80+0.07) 1.46(1.42+0.03) -21.9% 5326.5: simulated fetch 2.66(2.61+0.04) 1.47(1.43+0.04) -44.7% 5326.6: pack to file (bitmap) 2.74(2.62+0.12) 1.89(1.82+0.07) -31.0% Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>