Changed to a shifted mid=bid.rid representation for debugging

This only matters for developers, not users, but it still helps a lot to
get debug representations right.

Since the exact mid encoding depends on the block_size in an unintuitive
manner, it's tricky to render in a debug-friendly way that is useful
both with and without tools.

Previously, I avoided shifting the bid representation, since this would
be closer to the value in the device, but this hides the actual
structure of the mtree. Now the bid is shifted, showing the underlying
mtree/mdir structure, at the cost of needing to know the number of mbits
to encode the mid back into an integer.

So for example, on a device with 4KiB blocks, or 8 mbits:

  mid=1
  mid=258
  mid=515

Becomes:

  mid=0.1
  mid=1.2
  mid=2.3

This continues to make the mbits a more fundamental part of littlefs,
but that's probably just how that's going to be.

scripts/dbgmtree.py

@@ -761,6 +761,10 @@ def main(disk, mroots=None, *,
             f.seek(0, os.SEEK_END)
             block_size = f.tell()
 
+        # determine the number of mbits from the block_size, this is just
+        # for printing purposes
+        mbits = m.ceil(m.log2(block_size // 16))
+
         # before we print, we need to do a pass for a few things:
         # - find the actual mroot
         # - find the total weight
@@ -1303,8 +1307,8 @@ def main(disk, mroots=None, *,
                         if args.get('tree') or args.get('btree') else '',
                     '%*s %-22s%s' % (
                         w_width, '%d.%d-%d' % (
-                                mbid-max(mw-1, 0), rid-(w-1), rid)
-                            if w > 1 else '%d.%d' % (mbid-max(mw-1, 0), rid)
+                                mbid >> mbits, rid-(w-1), rid)
+                            if w > 1 else '%d.%d' % (mbid >> mbits, rid)
                             if w > 0 or i == 0 else '',
                         tagrepr(tag, w, len(data), j),
                         '  %s' % next(xxd(data, 8), '')
@@ -1360,8 +1364,9 @@ def main(disk, mroots=None, *,
                     treerepr(bid, w, bd, rid, 0, tag)
                         if args.get('tree') or args.get('btree') else '',
                     w_width, '' if i != 0
-                        else '%d-%d' % (bid-(w-1), bid) if w > 1
-                        else bid if w > 0
+                        else '%d-%d' % ((bid-(w-1)) >> mbits, bid >> mbits)
+                            if (w >> mbits) > 1
+                        else bid >> mbits if w > 0
                         else '',
                     tagrepr(tag, w if i == 0 else 0, len(data), None),
                     # note we render names a bit different here
@@ -1401,11 +1406,11 @@ def main(disk, mroots=None, *,
         #### actual debugging begins here
 
         # print some information about the mtree
-        print('mtree %s, rev %d, weight %d' % (
-            mroot.addr(), mroot.rev, mweight))
+        print('mtree %s, rev %d, weight %d.%d' % (
+            mroot.addr(), mroot.rev, mweight >> mbits, 1 << mbits))
 
         # print header
-        w_width = (m.ceil(m.log10(max(1, mweight)+1))
+        w_width = (m.ceil(m.log10(max(1, mweight >> mbits)+1))
             + 2*m.ceil(m.log10(max(1, rweight)+1))
             + 2)
         print('%-11s  %*s%-*s %-22s  %s' % (