i18n: add the tool to check Mercurial specific translation problems in *.po Existing tool like "msgfmt --check" can check typical translation problems (missing "%s" in msgstr, for example), but can't check Mercurial specific ones. For example, "msgfmt --check" can't check whether the translated string given to "ui.promptchoice()" is correct or not, even though problems like below cause run-time error or unexpected behavior: - less or more choices than msgid, - choices without '&', or - choices with '&' followed by none This patch adds the tool to check Mercurial specific translation problems in *.po files.