tersestatus: avoid modifying tersedict Turn dirnode's methods into generators which can be used to update "tersedict" in caller. So instead of passing the "tersedict" to be mutated here and there, it's now clearer where it is updated as it's purely a local variable to tersedir() function. While I was here, I renamed _processtersestatus to tersewalk and _addfilestotersed to iterfilepaths. Differential Revision: https://phab.mercurial-scm.org/D1043