fixup! fixup! fixup! ZBigFile: Add ZBlk format option 'h' (heuristic) (4)

'ZBlk0' and 'ZBlk1' always only change the block that's explicitly altered,
while zblk format 'auto' may also alter blocks different to the explicitly
changed blocks. In other words, as @kirr noted in nexedi/wendelin.core!20 (comment 198863),
format 'auto' break the invariant

    "transaction changes ZBlk objects only for modified blocks of array" [1]

Breaking this invariant leads to two failed WCFS tests -
'test_wcfs_crash_old_data' and 'test_wcfs_watch_vs_access'.

As suggested in nexedi/wendelin.core!20 (comment 198863)
instead of patching tests, we rather try not to break this invariant
with format 'auto'. This is exactly what this patch does.

[1] nexedi/wendelin.core!20 (comment 198863)

bigfile/file_zodb.py

@@ -602,10 +602,8 @@ class ZBigFile(LivePersistent):
         # append - migrate previously filled-up block to ZBlk0 for fast reads
         #        - for current block use ZBlk1 if the append is small and ZBlk0 otherwise
         if append:
-            if not zblk and blk > 0:  # is new zblk?
-                zblk_prev = self.blktab.get(blk-1)
-                if zblk_prev is not None  and  type(zblk_prev) is not ZBlk0:
-                    self._setzblk(blk-1, zblk_prev, zblk_prev.loadblkdata(), ZBlk0)
+            if len(new_data) >= (self.blksize - 6):  # is full?
+                return ZBlk0
             return ZBlk1 if small else ZBlk0
 
         # all other changes - use ZBlk1 if the change is small and ZBlk0 otherwise