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bitmap coverage is out-of-date with the ref tips, we spend more time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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ttaylorr

t/t5326-multi-pack-bitmaps.sh

@@ -283,4 +283,34 @@ test_expect_success 'pack.preferBitmapTips' '
 	)
 '
 
+test_expect_success 'hash-cache values are propagated from pack bitmaps' '
+	rm -fr repo &&
+	git init repo &&
+	test_when_finished "rm -fr repo" &&
+	(
+		cd repo &&
+
+		test_commit base &&
+		test_commit base2 &&
+		git repack -adb &&
+
+		test-tool bitmap dump-hashes >pack.raw &&
+		test_file_not_empty pack.raw &&
+		sort pack.raw >pack.hashes &&
+
+		test_commit new &&
+		git repack &&
+		git multi-pack-index write --bitmap &&
+
+		test-tool bitmap dump-hashes >midx.raw &&
+		sort midx.raw >midx.hashes &&
+
+		# ensure that every namehash in the pack bitmap can be found in
+		# the midx bitmap (i.e., that there are no oid-namehash pairs
+		# unique to the pack bitmap).
+		comm -23 pack.hashes midx.hashes >dropped.hashes &&
+		test_must_be_empty dropped.hashes
+	)
+'
+
 test_done