dirstate: move clear onto dirstatemap class A future diff will move the lazy loading aspect of dirstate to the dirstatemap class. This means it requires a slightly different strategy of clearing than just reinstantiating the object (since just reinstantiating the object will lazily load the on disk data again later instead of remaining permanently empty). So let's give it it's own clear function. Differential Revision: https://phab.mercurial-scm.org/D1252