discovery: drop findoutgoing and simplify findcommonincoming's api This is a long desired cleanup and paves the way for new discovery. To specify subsets for bundling changes, all code should use the heads of the desired subset ("heads") and the heads of the common subset ("common") to be excluded from the bundled set. These can be used revlog.findmissing instead of revlog.nodesbetween. This fixes an actual bug exposed by the change in test-bundle-r.t where we try to bundle a changeset while specifying that said changeset is to be assumed already present in the target. This used to still bundle the changeset. It no longer does. This is similar to the bugs fixed by the recent switch to heads/common for incoming/pull.