l.c.upgrade : module documentation

Provide Upgrader to upgrade any branch to a 2a format.

Provides special support for looms and subtree formats.

Repositories that have no tree references are always upgraded to the standard 2a format, even if they are in a subtree-supporting format. Repositories that actually have tree references are converted to RepositoryFormat2aSubtree.

Class	AlreadyUpgraded	Attempted to upgrade a branch that had already been upgraded.
Class	Upgrader	Upgrades branches to 2a-based formats if possible.