Meta's Sapling source-code management system

Hmm. We use topic branches for *all* development (there are a few exceptions; mainly automatic development version number bumps, but nothing manual). Branches for releases are `-s ours` merged into more recent branches (this preserves an "all history is reachable from HEAD" property and means we can trivially resurrect any old branch for maintenance as needed). But we also have strict vendoring rules and mangle everything to avoid conflicts with anything that could be loaded in the same process (such is life when you make SDK-like things, not end-user products).