Code review comment for lp:~nathanael-naeri/deja-dup/fix-1549776

This looks fine, thank you so much!

Ideally there'd be a test exercising the new functionality. But I don't want to block the improvement on that.

And ideally this whole code path would be a bit more robust. Like we replace $USER, but do so even if the user uses $USERSTRING or some such nonsense. But that's a separate patch and perhaps this feature doesn't really need an industrial-strength parser.