_findTranslationMessage can return a TM diverged in another template

POTMsgSet._findTranslationMessage can return a diverged message from another template and try to use that, when that should happen in only very specific set of conditions (i.e. when we are making that a shared translation in the new template).

This is similar to the export problem, but it's slightly more complex: we need to differentiate between making a shared translation or making a diverged translation to decide whether we want to look into other diverged TMs as well, or just for (TM.potemplate IS NULL or TM.potemplate=pofile.potemplate).