t7704: demonstrate failure with once-cruft objects above the geometric split

Add a test demonstrating a case where geometric repacking fails to
produce a pack with full object closure, thus making it impossible to
write a reachability bitmap.

Mark the test with 'test_expect_failure' for now. The subsequent commit
will explain the precise failure mode, and implement a fix.

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

Author: ttaylorr

t/t7704-repack-cruft.sh

@@ -869,4 +869,26 @@ test_expect_success 'repack --write-midx includes cruft when already geometric'
 	)
 '
 
+test_expect_failure 'repack rescues once-cruft objects above geometric split' '
+	git config repack.midxMustContainCruft false &&
+
+	test_commit reachable &&
+	test_commit unreachable &&
+
+	unreachable="$(git rev-parse HEAD)" &&
+
+	git reset --hard HEAD^ &&
+	git tag -d unreachable &&
+	git reflog expire --all --expire=all &&
+
+	git repack --cruft -d &&
+
+	echo $unreachable | git pack-objects .git/objects/pack/pack &&
+
+	test_commit new &&
+
+	git update-ref refs/heads/other $unreachable &&
+	git repack --geometric=2 -d --write-midx --write-bitmap-index
+'
+
 test_done