Always log mbid=-1 for mroots and inlined mdirs

So mbid=0 now implies the mdir is not inlined.

Downsides:

- A bit more work to calculate
- May lose information due to masking everything when mtree.weight==0
- Risk of confusion when in-lfs.c state doesn't match (mbid=-1 is
  implied by mtree.weight==0)

Upsides:

- Includes more information about the topology of the mtree
- Avoids multiple dbgmbids for the same physical mdir

Also added lfsr_dbgmbid and lfsr_dbgmrid to help make logging
easier/more consistent.

And updated dbg scripts.

scripts/dbgmtree.py

@@ -1700,11 +1700,10 @@ class Mtree:
                 else:
                     return None
 
-            mdir = Mdir(0, self.mroot)
             if path:
-                return mdir, path_
+                return self.mroot, path_
             else:
-                return mdir
+                return self.mroot
 
         # mtree? lookup in mtree
         else:
@@ -2074,11 +2073,10 @@ class Mtree:
 
         # no mtree? must be inlined in mroot
         if self.mtree is None:
-            mdir = Mdir(0, self.mroot)
             if path:
-                return mdir, path_
+                return self.mroot, path_
             else:
-                return mdir
+                return self.mroot
 
         # mtree? find name in mtree
         else: