t5319: don't write MIDX bitmaps in t5319

This test is specifically about generating a midx still respecting a
pack-based bitmap file. Generating a MIDX bitmap would confuse the test.
Let's override the 'GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP' variable to
make sure we don't do so.

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

t/t5319-multi-pack-index.sh

@@ -532,7 +532,8 @@ test_expect_success 'repack preserves multi-pack-index when creating packs' '
 compare_results_with_midx "after repack"
 
 test_expect_success 'multi-pack-index and pack-bitmap' '
-	git -c repack.writeBitmaps=true repack -ad &&
+	GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP=0 \
+		git -c repack.writeBitmaps=true repack -ad &&
 	git multi-pack-index write &&
 	git rev-list --test-bitmap HEAD
 '