InferCommit gets a 40-character rev hash from the git clone enclosing the working dir.

InferRepo gets a Sourcegraph-friendly repo name from the git clone enclosing the working dir.

InferRoot gets the path relative to the root of the git clone enclosing the given file path.

SanitizeRoot removes redundant paths from the given root and replaces references to the root of th repo with an empty string.