Translation script should ignore the source translation file

Use case happening today: I'm releasing a bug fix version where some changes of a source translation were not cherry-picked (because depending on a feature not cherry-picked).
The apply_translation script did provided those changes among the translations change.

It doesn't really make sense to automatically include those because only committers can actually update those files: they cannot be changed through l10n platform.

Now if we do ignore them, the translation script could include translations change related to the modified source, without including the modified source. Which might lead to some inconsistency between english and other languages. IMO that's less risky than always ignoring the source file, since it's supposed to be "the truth" for a given release.