export file renaming

This is seriously super hairy. It has to handle interrupted exports,
which may be resumed with the same or a different tree. It also has to
recover from export conflicts, which could cause the wrong content
to be renamed to a file.

I think this works, or is close to working. See the update to the design
for how it works.

This is definitely not optimal, in that it does more renames than are
necessary. It would probably be worth finding the keys that are really
renamed and only renaming those. But let's get the "simple" approach to
work first..

This commit was supported by the NSF-funded DataLad project.

Types/Remote.hs

@@ -178,5 +178,7 @@ data ExportActions a = ExportActions
 	-- Throws an exception if the remote cannot be accessed.
 	, checkPresentExport :: Key -> ExportLocation -> a Bool
 	-- Renames an already exported file.
+	-- This may fail, if the file doesn't exist, or the remote does not
+	-- support renames.
 	, renameExport :: Key -> ExportLocation -> ExportLocation -> a Bool
 	}