Merge branch 'nd/upload-pack-shallow'

Merge branch 'nd/upload-pack-shallow'

Serving objects from a shallow repository needs to write a
temporary file to be used, but the serving upload-pack may not have
write access to the repository which is meant to be read-only.

Instead feed these temporary shallow bounds from the standard input
of pack-objects so that we do not have to use a temporary file.

* nd/upload-pack-shallow:
  upload-pack: send shallow info over stdin to pack-objects