Offline work when bugfixing

Hello,

Let's imagine a component is 0% translated and we have translator T1 working online and translator T2 working offline (using translation download & upload). It works fine I guess.
But now let's say the component is 100% translated and T1 and T2 are both bugfixing: in that case I think the workflow is broken, because when T2 chooses whether to check the "Overwrite" checkbox at upload, he actually chooses between discarding his work (upload does nothing) or discarding T1's work.

I think it would be useful to have a third choice which would yield a kind of auto-diff at upload, to upload only what's changed.
This could probably be achieved by conveying a git hash in the translation header at download (@samhocevar's suggestion) (not sure how that would work for other vcs, but hopefully similarly) (it would force a commit during download I guess)
Thanks!
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