This change adds a way for Review Bot tools to operate with an entire checkout

of a (git) repository. This requires a fair amount of configuration on the

worker side, which I'm planning to document more thoroughly in a forthcoming

user manual.
When one or more repositories is configured, the worker will listen on a new

set of queues (the product of the list of repositories and the available tools

which can run on full repos). This way, a worker will only pick up tasks for

repositories which it has available.
When a change using a repository is triggered for review, the repository is

either cloned (if it doesn't exist), or fetched. This repo is then

shallow-cloned to the desired parent revision and the patched files are

downloaded. The tool can then do whatever it needs with the repo and post its

review.

Verified that clone/fetch and the shallow cloned worked as expected.
Used this in conjunction with a Clang Static Analyzer tool.