This extension is not distributed with Mercurial.
Author: Durham Goode
Current maintainer: Facebook
The remotefilelog extension allows Mercurial to clone shallow copies of a repository such that all file contents are left on the server and only downloaded on demand by the client. This greatly speeds up clone and pull performance for repositories that have long histories or that are growing quickly.
In addition, the extension allows using a caching layer (such as memcache) to serve the file contents, thus providing better scalability and reducing server load.
This extension is still being developed and frequently changes. See the bitbucket page for the most recent documentation.