Repository export task disk free check does not take previous exports into account

The repository export task does a check of free disk space in the target directory before starting. It compares this against the total size of all of the assets in the source repository.

This works fine for an initial export.

The problem is, after repeated exports are run we're always requiring free space equivalent to the entire repository size, even though subsequent exports will only export the delta between the current and previous exports. This causes the export task to fail when it should not, and requires allocation of more disk space than is needed for the export to succeed.