Merge Issue with CollapsingMergeTree Leading to Incomplete Granules Error

In rare case, merge for CollapsingMergeTree may create granule with index_granularity + 1 rows. Because of this, internal check, added in #18928 (affects 21.2 and 21.3), may fail with error Incomplete granules are not allowed while blocks are granules size. This error did not allow parts to merge. #21976 (Nikolai Kochetov).