[PATCH 3/4] repack: retain cruft packs in MIDXs after incremental repacks

[Taylor Blau <ttaylorr@openai.com>]

An incremental repack can write a commit and tree into a new pack while
leaving objects they reach in an existing cruft pack. For example, a
commit can make a previously unreachable blob reachable again. Since
'repack' will invoke 'pack-objects' with '--incremental', it will not
copy the blob out of its cruft pack.

When the 'repack.midxMustContainCruft' configuration is set to "false",
writing the first MIDX after such a repack may omit that cruft pack. The
new pack bypasses the `!names.nr` fallback, and there are no previous
MIDX packs for `midx_has_unknown_packs()` to check. Selecting the new
commit for bitmap coverage then fails because its reachable objects are
not all in the MIDX.

The omission dates all the way back to 5ee86c273bf (repack: exclude
cruft pack(s) from the MIDX where possible, 2025-06-23). It relies on
geometric repacking to copy once-cruft objects with
'--stdin-packs=follow'. However, an ordinary incremental repack makes no
such guarantee. Require the MIDX to include cruft packs in that case,
even when a new pack was written.

Exercise this with the existing fixture that makes a cruft commit
reachable again and adds a new (unpacked) commit on top, and ensure that
the incremental repack is able to successfully write a reachability
bitmap.

Signed-off-by: Taylor Blau <ttaylorr@openai.com>
---
 builtin/repack.c        |  6 ++++++
 t/t7704-repack-cruft.sh | 11 +++++++++++
 2 files changed, 17 insertions(+)

diff --git a/builtin/repack.c b/builtin/repack.c
index c4360382c1f..b7596d488da 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -539,6 +539,12 @@ int cmd_repack(int argc,
 			strvec_push(&cmd.args, "--stdin-packs=follow");
 		strvec_push(&cmd.args, "--unpacked");
 	} else {
+		/*
+		 * Incremental repacks do not copy already-packed objects,
+		 * so cruft packs may be required to form a reachability
+		 * closure for the MIDX.
+		 */
+		midx_must_contain_cruft = 1;
 		strvec_push(&cmd.args, "--unpacked");
 		strvec_push(&cmd.args, "--incremental");
 	}
diff --git a/t/t7704-repack-cruft.sh b/t/t7704-repack-cruft.sh
index b49f22878f7..f7f83e70ffe 100755
--- a/t/t7704-repack-cruft.sh
+++ b/t/t7704-repack-cruft.sh
@@ -787,6 +787,17 @@ test_expect_success 'geometric repack rescues descendants of loose trees' '
 	)
 '
 
+test_expect_success 'incremental repack includes cruft for MIDX bitmaps' '
+	setup_cruft_exclude_tests incremental-cruft &&
+	(
+		cd incremental-cruft &&
+
+		GIT_TEST_MULTI_PACK_INDEX=0 \
+		git repack -d --write-midx --write-bitmap-index &&
+		git rev-list --test-bitmap HEAD
+	)
+'
+
 test_expect_success 'repack --write-midx includes cruft when instructed' '
 	setup_cruft_exclude_tests exclude-cruft-when-instructed &&
 	(
-- 
2.56.0.4.gbee41d2fc68