Export conflicts happen whenever two repos export different trees 
to the same special remote.

Some conflicts that seem trivial currently involve unnecessary unexporting
and re-uploading in their resolution.

For example, if A exports a tree containing `[foo]`, and B exports a tree
containing `[foo, bar]`, bar gets unexported when resolving the conflict.

Bear in mind that A could have first exported a tree containing `[foo, bar]`
then B exported a tree containing `[foo, bar]`, and then A exported
`[foo]`, so unexported bar.

So, if one exported tree is a subset of the other, it's not necessary to
unexport files added by the other tree. It's sufficient to check that files
are present in the export and upload any that are missing. --[[Joey]]

[[!tag confirmed]]

> I proved this is not a safe optimisation, so [[wontfix|done]] --[[Joey]]
