dirstate: avoid reading the map when possible (issue5713) (issue5717) Before the recent refactor, we would not load the entire map until it was accessed. As part of the refactor, that got lost and even just trying to load the dirstate parents would load the whole map. This caused a perf regression (issue5713) and a regression with static http serving (issue5717). Making it lazy loaded again fixes both. Differential Revision: https://phab.mercurial-scm.org/D1253

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

internal-doc: document the config register mechanism This explains the various usage and feature of the config register introduced in Mercurial 4.3 and 4.4.