export: fix multi-file delete bug

export: fix multi-file delete bug

export: Fix a bug that left a file on a special remote when two files with
the same content were both deleted in the exported tree.

Case of the wrong data structure leading to the wrong result.
The DiffMap now contains all the old filenames, and all the new filenames.

Note that, when 2 files with the same content are both renamed,
it only renames the first, but deletes and re-exports the second.
Improving that is possible, but it would need to use a different temporary
filename. Anyway, that is an unusual case, and there are known to be other
unusual cases where export does not rename with maximum efficiency, IIRC.
(Or maybe this is the case that I remember?)

Sponsored-by: Dartmouth College's OpenNeuro project