This patch introduces more intelligent Mercurial metadata lookup.

Before, the approach was to have rbtools shell out to Mercurial every

time it needed data. This was prone to inefficiencies.
In the new world, we have a class that holds all known state about

the Mercurial repository and provides a just-in-time method to resolve

missing metadata on access.
Not everything uses the new mechanism yet. But I think this patch

represents a good half-way point from which additional improvements can

be built upon.

Test suite passes.