Avoid negative block sizes in lazyparser.
This fixes a bug introduced by changeset baa3873eb387 .
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -143,7 +143,7 @@ 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