Avoiding writing files larger than a specified size is useful on certian
things. For example, box.com has a file size limit of 100 mb. Could also
be useful on really crappy removable media.
Fully tested and working, including resuming and encryption. (Though not
resuming when sending *with* encryption; gpg doesn't produce identical
output each time.)
Uses same layout as the directory special remote and the .git/annex/objects/
directory.
