Copy-on-write fuse filesystem

to check out a whole group of repositories without using more disk space

http://m.cacm.acm.org/magazines/2016/7/204032-why-google-stores-billions-of-lines-of-code-in-a-single-repository/fulltext "Most developers access Piper through a system called Clients in the Cloud, or CitC, which consists of a cloud-based storage backend and a Linux-only FUSE13 file system. Developers see their workspaces as directories in the file system, including their changes overlaid on top of the full Piper repository. CitC supports code browsing and normal Unix tools with no need to clone or sync state locally. Developers can browse and edit files anywhere across the Piper repository, and only modified files are stored in their workspace. This structure means CitC workspaces typically consume only a small amount of storage (an average workspace has fewer than 10 files) while presenting a seamless view of the entire Piper codebase to the developer."