Fixed coloring conflicts in rbyd tree renderers

A bit of a hack, but rather than handling conditional alt branches, our
dbg rbyd tree renderers just represent single-pointer alts as an alt
with both branches pointing to the place.

Unfortunately, the two branches technically have different colors. This
resulted in a bit of contention when chosing how to color the tree.
Basically Python's dict ordering would determine which color won.

Which was a bit confusing when dbgrbyd.py displayed different tree
colorings for the same rbyd. dbgrbyd.py should be idempotent!

This is solved by adding another hack to check explicitly for
same-destination branches.

scripts/dbgmtree.py

@@ -563,12 +563,13 @@ class Rbyd:
                 d=t_depth-1 - alt['h'],
                 c=alt['c'],
             ))
-            tree.add(TBranch(
-                a=alt['nft'],
-                b=alt['ft'],
-                d=t_depth-1 - alt['h'],
-                c='b',
-            ))
+            if alt['ft'] != alt['nft']:
+                tree.add(TBranch(
+                    a=alt['nft'],
+                    b=alt['ft'],
+                    d=t_depth-1 - alt['h'],
+                    c='b',
+                ))
 
         return tree, t_depth