Dropped separate BTREE/BRANCH encodings

There is a bit of redundancy here, as we already know the weights of
btree's inner-branches from their parents. But in theory sharing the
same encoding for both the top level btree reference and inner-branches
should offer more chance for deduplication and hopefully less code.

This also moves some members around in the btree encoding so that the
redund blocks are at the beginning. This _might_ simplify decoding of
the variable-length redund blocks at some point.

Current btree encoding:

  .----+----+----+----.
  |       blocks    ...  redund leb128s (1-20 bytes)
  :                   :
  |----+----+----+----|
  |       trunk     ...  1 leb128 (1-5 bytes)
  |----+----+----+----|
  |       weight    ...  1 leb128 (1-5 bytes)
  |----+----+----+----|
  |       cksum       |  1 le32 (4 bytes)
  '----+----+----+----'

This also partially reverts some tag name changes:

- BNAME -> BRANCH
- DMARK -> BOOKMARK

scripts/dbgmtree.py

@@ -14,8 +14,8 @@ TAG_SUPERCONFIG = 0x0004
 TAG_GSTATE      = 0x0100
 TAG_GRM         = 0x0100
 TAG_NAME        = 0x0200
-TAG_BNAME       = 0x0200
-TAG_DMARK       = 0x0201
+TAG_BRANCH      = 0x0200
+TAG_BOOKMARK    = 0x0201
 TAG_REG         = 0x0202
 TAG_DIR         = 0x0203
 TAG_STRUCT      = 0x0300
@@ -25,8 +25,7 @@ TAG_BTREE       = 0x030c
 TAG_MDIR        = 0x0311
 TAG_MTREE       = 0x0314
 TAG_MROOT       = 0x0318
-TAG_BRANCH      = 0x031c
-TAG_DID         = 0x0320
+TAG_DID         = 0x031c
 TAG_UATTR       = 0x0400
 TAG_SATTR       = 0x0500
 TAG_ALT         = 0x4000
@@ -106,18 +105,13 @@ def frommdir(data):
         d += d_
     return blocks
 
-def frombranch(data):
+def frombtree(data):
     d = 0
     block, d_ = fromleb128(data[d:]); d += d_
     trunk, d_ = fromleb128(data[d:]); d += d_
-    cksum = fromle32(data[d:]); d += 4
-    return block, trunk, cksum
-
-def frombtree(data):
-    d = 0
     w, d_ = fromleb128(data[d:]); d += d_
-    block, trunk, cksum = frombranch(data[d:])
-    return w, block, trunk, cksum
+    cksum = fromle32(data[d:]); d += 4
+    return block, trunk, w, cksum
 
 def popc(x):
     return bin(x).count('1')
@@ -153,8 +147,8 @@ def tagrepr(tag, w, size, off=None):
             size)
     elif (tag & 0xff00) == TAG_NAME:
         return '%s%s %d' % (
-            'bname' if tag == TAG_BNAME
-                else 'dmark' if tag == TAG_DMARK
+            'branch' if tag == TAG_BRANCH
+                else 'bookmark' if tag == TAG_BOOKMARK
                 else 'reg' if tag == TAG_REG
                 else 'dir' if tag == TAG_DIR
                 else 'name 0x%02x' % (tag & 0xff),
@@ -168,7 +162,6 @@ def tagrepr(tag, w, size, off=None):
                 else 'mdir' if tag == TAG_MDIR
                 else 'mtree' if tag == TAG_MTREE
                 else 'mroot' if tag == TAG_MROOT
-                else 'branch' if tag == TAG_BRANCH
                 else 'did' if tag == TAG_DID
                 else 'struct 0x%02x' % (tag & 0xff),
             ' w%d' % w if w else '',
@@ -527,7 +520,7 @@ class Rbyd:
                     rid_, w = rid__, w_
 
                 # catch any branches
-                if tag == TAG_BRANCH:
+                if tag == TAG_BTREE:
                     branch = (tag, j, d, data)
 
                 tags.append((tag, j, d, data))
@@ -539,7 +532,7 @@ class Rbyd:
             if branch is not None and (
                     not depth or depth_ < depth):
                 tag, j, d, data = branch
-                block, trunk, cksum = frombranch(data)
+                block, trunk, _, cksum = frombtree(data)
                 rbyd = Rbyd.fetch(f, block_size, block, trunk)
 
                 # corrupted? bail here so we can keep traversing the tree
@@ -634,7 +627,7 @@ class Rbyd:
                     ))
 
                 d_ += max(bdepths.get(d, 0), 1)
-                leaf = (bid-(w-1), d, rid-(w-1), TAG_BRANCH)
+                leaf = (bid-(w-1), d, rid-(w-1), TAG_BTREE)
 
         # remap branches to leaves if we aren't showing inner branches
         if not inner:
@@ -813,7 +806,7 @@ def main(disk, mroots=None, *,
         if not args.get('depth') or mdepth < args.get('depth'):
             done, rid, tag, w, j, d, data, _ = mroot.lookup(-1, TAG_MTREE)
             if not done and rid == -1 and tag == TAG_MTREE:
-                w, block, trunk, cksum = frombtree(data)
+                block, trunk, w, cksum = frombtree(data)
                 mtree = Rbyd.fetch(f, block_size, block, trunk)
 
                 mweight = w
@@ -1482,7 +1475,7 @@ def main(disk, mroots=None, *,
         if not args.get('depth') or mdepth < args.get('depth'):
             done, rid, tag, w, j, d, data, _ = mroot.lookup(-1, TAG_MTREE)
             if not done and rid == -1 and tag == TAG_MTREE:
-                w, block, trunk, cksum = frombtree(data)
+                block, trunk, w, cksum = frombtree(data)
                 mtree = Rbyd.fetch(f, block_size, block, trunk)
 
                 # traverse entries