fix #1223, crash+inefficiency during repair due to read overrun

fix #1223, crash+inefficiency during repair due to read overrun

* repairer (really the uploader) reads beyond end of input file (Uploadable)
* new-downloader does not tolerate overreads
* uploader does lots of tiny reads (inefficient)

This fixes the last two. The uploader still does a single overread at the end
of the input file, but now that's ok so we can leave it in place. The
uploader now expects the Uploadable to behave like a normal disk
file (reading beyond EOF will return less data than was asked for), and now
the new-downloadable behaves that way.