tests: ignore test-storage when using Rust This is only relevant for Python code and the SQLite backend, which is in a half-abandoned state.

rust-index: optimize find_gca_candidates() on less than 8 revisions This is expected to be by far the most common case, given that, e.g., merging involves using it on two revisions. Using a `u8` as support for the bitset obviously divides the amount of RAM needed by 8. To state the obvious, on a repository with 10 million changesets, this spares 70MB. It is also possible that it'd be slightly faster, because it is easier to allocate and provides better cache locality. It is possible that some exhaustive listing of the traits implemented by `u8` and `u64` would avoid the added duplication, but that can be done later and would need a replacement for the `MAX` consts.

rust-index: simplification in find_gca_candidates() `parent_seen` can be made a mutable ref, making this part more obvious, not needing to be commented so much. The micro-optimization of avoiding the union if `parent_seen` and `current_seen` agree is pushed down in the `union()` method of the fast, `u64` based bit set implementation (in case it matters).

rust-index: avoid double negation in find_gca_candidates()