i18n: cache translated messages per encoding This is a simpler workaround alternative to D958, "i18n: clean msgcache when encoding changes." The cache won't be bloated unless you run tons of commands with different --encoding options on command server, or serve many repositories of different web.encoding options on hgweb. The test was originally written by Jun Wu. Differential Revision: https://phab.mercurial-scm.org/D1053