Avoid negative block sizes in lazyparser.

This fixes a bug introduced by changeset baa3873eb387 .

mercurial/revlog.py

@@ -143,7 +143,7 b' class lazyparser(object):'
                 # the revlog may have grown since we've started running,
                 # but we don't have space in self.index for more entries.
                 # limit blocksize so that we don't get too much data.
-                blocksize = self.datasize - blockstart
+                blocksize = max(self.datasize - blockstart, 0)
             data = self.dataf.read(blocksize)
         lend = len(data) / self.s
         i = blockstart / self.s