Draft: Preparatory pack-objects cache changes
This MR collects a few commits we need for !3668 (closed) and gitlab-com/gl-infra/scalability#1126 (closed).
- A mechanism to turn a stream of side-band64k packets into an io.Reader (SingleBandReader).
- A change in the cache file format so that the exit status of pack-objects (true/false) is reflected in the cache files: this allows us to later implement PackObjectsHook as simply streaming the cache file.
Edited by Jacob Vosmaer