actions: introduce function to calculate downgrades
An upgrade operation can also downgrade/remove some format variants. Before this
patch there was no clean way to find out all such variants which will be
removed. This patch adds a function for that.
It will be used in next patch.
Differential Revision: https://phab.mercurial-scm.org/D9618
# rhg
This project provides a fastpath Rust implementation of the Mercurial (`hg`)
version control tool.