manifest: adds manifestctx.readfast This adds a copy of manifest.readfast to manifestctx.readfast and adds a consumer of it. It currently looks like duplicate code, but a future patch causes these functions to diverge as tree concepts are added to the tree version.

manifest: add manifestctx.readdelta() This adds an implementation of readdelta to the new manifestctx class and adds a couple consumers of it. This currently appears to have some duplicate code, but future patches cause this function to diverge when things like "shallow" are introduced.