rhg: add a limited `rhg cat -r` subcommand
It only supports revision specification (rev or full hash) and the list of files to cat. Differential Revision: https://phab.mercurial-scm.org/D9052
It only supports revision specification (rev or full hash) and the list of files to cat. Differential Revision: https://phab.mercurial-scm.org/D9052