Function hg::dagops::retain_heads

pub fn retain_heads(
    graph: &impl Graph, 
    revs: &mut HashSet<Revision>
) -> Result<(), GraphError>

Retain in revs only its relative heads.

This is an in-place operation, so that control of the incoming set is left to the caller.

• a direct Python binding would probably need to build its own HashSet from an incoming iterable, even if its sole purpose is to extract the heads.
• a Rust caller can decide whether cloning beforehand is appropriate

Performance notes

Internally, this function will store a full copy of revs in a Vec.