commitextras: make sure keys contains ascii letters, numbers, '_' and '-' only

phabricator: convert unicode to binary when writing patches This is a quick fix to make `hg phabread D189` work. It seems we might want to replace all `r''` to `u''`, and add more `encoding.*to*` to be more explicit when interacting with `json` module. Differential Revision: https://phab.mercurial-scm.org/D192