pack-bitmap-write: rebuild using new bitmap when remapping

`git repack` is about to learn a new `--filter=<filter-spec>` option and
we will want to check that this option is incompatible with
`--write-bitmap-index`.

Unfortunately it appears that a test like:

test_expect_success '--filter fails with --write-bitmap-index' '
       test_must_fail \
               env GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP=0 \
               git -C bare.git repack -a -d --write-bitmap-index --filter=blob:none
'

sometimes fail because when rebuilding bitmaps, it appears that we are
reusing existing bitmap information. So instead of detecting that some
objects are missing and erroring out as it should, the
`git repack --write-bitmap-index --filter=...` command succeeds.

Let's fix that by making sure we rebuild bitmaps using new bitmaps
instead of existing ones.

Helped-by: Taylor Blau <[email protected]>
Signed-off-by: Christian Couder <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: chriscool

pack-bitmap-write.c

@@ -413,15 +413,19 @@ static int fill_bitmap_commit(struct bb_commit *ent,
 
 		if (old_bitmap && mapping) {
 			struct ewah_bitmap *old = bitmap_for_commit(old_bitmap, c);
+			struct bitmap *remapped = bitmap_new();
 			/*
 			 * If this commit has an old bitmap, then translate that
 			 * bitmap and add its bits to this one. No need to walk
 			 * parents or the tree for this commit.
 			 */
-			if (old && !rebuild_bitmap(mapping, old, ent->bitmap)) {
+			if (old && !rebuild_bitmap(mapping, old, remapped)) {
+				bitmap_or(ent->bitmap, remapped);
+				bitmap_free(remapped);
 				reused_bitmaps_nr++;
 				continue;
 			}
+			bitmap_free(remapped);
 		}
 
 		/*