scripts: dbgmtree.py: Tightened dynamic mrid width

This requires an additional traversal of the mtree just to precalculate
the mrid width (mbits provides an upper-bound, but the actual number of
mrids in any given mdir may be much less), but it makes the output look
nicer.

scripts/dbgmtree.py

@@ -2324,7 +2324,10 @@ def main(disk, mroots=None, *,
         # dynamically size the id field
         w_width = max(
                 mt.ceil(mt.log10(max(1, mtree.mbweight_())+1)),
-                mt.ceil(mt.log10(max(1, mtree.mrweight_())+1)),
+                mt.ceil(mt.log10(max(1, max(
+                    mdir.weight for mdir in mtree.mdirs(
+                            depth=args.get('depth'))
+                        if isinstance(mdir, Mdir))))+1),
                 # in case of -1.-1
                 2)