fix non-moving gc heap space requirements misestimate (#20475)
This fixes #20475 (closed).
When using the non-moving gc, we will have space requirements comparable to the compacting gc not the copying gc.
Due to a large amount of spam we do not allow new users to create repositories, they are "external" users. If you are a new user and want to create a repository, for example for forking GHC, open a new issue on ghc/ghc using the "get-verified" issue template
This fixes #20475 (closed).
When using the non-moving gc, we will have space requirements comparable to the compacting gc not the copying gc.