I'm seeing the github lfs server request an upload of an object that has
already been uploaded to it before. Probably because they offload
storage to S3 and so skipped the overhead of checking for an unncessary
upload.
Let's keep this entirely pure.
git-annex has its own facilities for running a ssh command, that make it
respect various config settings, and cache connections, etc. So better
not to have the library run ssh itself.
