This resolves a long-standing issue in development where `mach artifact` (and therefore `mach bootstrap`) would fail unpredictably if you had dirty, but ignored, files in your checkout. Resolving this problem often required unwieldy `hg purge`/`git ignore` incantations that are easy to get wrong.
This patch addresses the problem by doing what we "should" have been doing all along, and consulting the VCS to list tracked files rather than listing EVERY file on disk and applying heuristics to determine whether they should be included in the hash.
Differential Revision: https://phabricator.services.mozilla.com/D86780
