ttag update adds plural forms to existing translations that shouldn't exist

[kevinlul]

When ttag update first detects a translation and adds it to the .po file, it adds the correct number of msgstr[] "" entries for the number of plural forms in the locale, as specified in the Plural-Forms header. However, once the translation is already in the file, subsequent runs seem to add msgstr[1] "" anyway, for example with zh-CN and ko locales, even though these languages don't have plural forms.