POFile export: move divergence handling into SQL

[Jeroen T. Vermeulen]

POFileToTranslationFileDataAdapter._getMessages handles divergence as follows:
 • The methods that fetch translation rows sort diverged messages to the front.
 • _getMessages keeps a set of diverged messages it's already seen.
 • When a non-diverged message is for a POTMsgSet for which it's already seen a diverged message, the non-diverged message is skipped.

The visible side effect of this is that exports don't strictly follow message-sequence order. Also, it means retrieving more rows than necessary.

Instead, we could move this into the SQL using "DISTINCT ON." That way we only fetch the applicable message: diverged if there is one, otherwise the shared one.

[Данило Шеган (danilo)]

The order messages are returned in should be by translationtemplateitem.sequence, translationmessage.potemplate, so order should still be correct. If it isn't, we can fix it without fixing this bug, if I am reading it correctly.

We should still move it to SQL, though, so that's where we can make the code look nicer.

[Jeroen T. Vermeulen (jtv)]

I concur. Right now, sharing affects the order in which messages are exported and it's confusing.