this only ever worked for empty uploads, and there it worked only by
complete accident: curl was asked to send more data than the wrapper
would provide, which curl would not like and report as an error. the
error would cause a retry with even less data to send, until finally
failing by running into the retry limit. let's just forbid all this.
Change-Id: I229a94b3b8b33e2c6cdb8ea19edd57cd6740e6c6
eldritch horrors
5d02800e57