Updated comments to prefer "canonical checksum" for rbyd checksums

I think this describes the goal of the non-perturbed rbyd checksums
decently. At the very least it's less wrong that "data checksum", and
calling it the "metadata checksum" would just be confusing. (Would our
commit checksum be the "metametadata checksum" then?)

scripts/dbgmtree.py

@@ -401,7 +401,7 @@ class Rbyd:
 
                 # end of trunk?
                 if not tag & TAG_ALT:
-                    # update data checksum
+                    # update canonical checksum
                     cksum_ = cksum__
                     # update trunk/weight unless we found a shrub or an
                     # explicit trunk (which may be a shrub) is requested