rust-index: implement common_ancestors_heads() and ancestors() The only differences betwwen `common_ancestors_heads()` and `find_gca_candidates()` seems to be that: - the former accepts "overlapping" input revisions (meaning with duplicates). - limitation to 24 inputs (in the C code), that we translate to using the arbitrary size bit sets in the Rust code because we cannot bail to Python. Given that the input is expected to be small in most cases, we take the heavy handed approach of going through a HashSet and wait for perfomance assessment In case this is used via `hg-cpython`, we can anyway absorb the overhead by having `commonancestorheads` build a vector of unique values directly, and introduce a thin wrapper over `find_gca_candidates`, to take care of bit set type dispatching only. As far as `ancestors` is concerneed, this is just chaining `common_ancestors_heads()` with `find_deepest_revs`.