Non-native Mercurial: allow Commit resolution from Mercurial hash
With #421 (closed), we made Commit resolution from Git hash work to ease transition to Mercurial native repositories.
It is no more complicated to do the same in reverse: resolve Commits from Mercurial hash for non-native (hg-git based) repositories. It will help us introduce some uniform schemes in advance of the complete migration of all repositories as native ones, and it already has direct applications such as #394 (closed).