repack: respect kept objects with '--write-midx -b'

Historically, we needed a single packfile in order to have reachability
bitmaps. This introduced logic that when 'git repack' had a '-b' option
that we should stop sending the '--honor-pack-keep' option to the 'git
pack-objects' child process, ensuring that we create a packfile
containing all reachable objects.

In the world of multi-pack-index bitmaps, we no longer need to repack
all objects into a single pack to have valid bitmaps. Thus, we should
continue sending the '--honor-pack-keep' flag to 'git pack-objects'.

The fix is very simple: only disable the flag when writing bitmaps but
also _not_ writing the multi-pack-index.

This opens the door to new repacking strategies that might want to keep
some historical set of objects in a stable pack-file while only
repacking more recent objects.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

builtin/repack.c

@@ -693,7 +693,7 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 		write_bitmaps = 0;
 	}
 	if (pack_kept_objects < 0)
-		pack_kept_objects = write_bitmaps > 0;
+		pack_kept_objects = write_bitmaps > 0 && !write_midx;
 
 	if (write_bitmaps && !(pack_everything & ALL_INTO_ONE) && !write_midx)
 		die(_(incremental_bitmap_conflict_error));

t/t7700-repack.sh

@@ -372,4 +372,19 @@ test_expect_success '--write-midx with preferred bitmap tips' '
 	)
 '
 
+test_expect_success '--write-midx -b packs non-kept objects' '
+	git init midx-kept &&
+	test_when_finished "rm -fr midx-kept" &&
+	(
+		cd midx-kept &&
+		test_commit_bulk 100 &&
+		GIT_TRACE2_EVENT="$(pwd)/trace.txt" \
+			git repack --write-midx -a -b &&
+		cat trace.txt | \
+			grep \"event\":\"start\" | \
+			grep pack-objects | \
+			grep \"--honor-pack-keep\"
+	)
+'
+
 test_done