nodes: expand/comment the magic nodes so they are more easily searchable We just encountered `000000000000modified`, and it was quite annoying to search for these, even though I knew they existed. For those that don't know that they exist, this is essentially impossible to search for :) (Technically we encountered it in its hex form, 3030303030303030303030306d6f646966696564, so I'm adding comments with those forms in case that's helpful to people in the future). Differential Revision: https://phab.mercurial-scm.org/D4331