treeify: track existence by node refs, rather than paths
treeify: track existence by node refs, rather than paths

Should have done this long ago. Now it doesn't rely on input and output
paths being the same anymore, because it's /not what we want to check/
in the first place...

grawity/hacks,grawity/hacks,grawity/hacks,grawity/hacks,grawity/hacks,grawity/hacks,grawity/hacks