Use unsigned version format.

This way can use one additional bit, and when encountering invalid revlogs
with the first bit set don't produce python warnings or strange error messages.

mercurial/revlog.py

@@ -81,7 +81,7 b' v0shaoffset = 56'
 # 32 bytes: nodeid
 indexformatng = ">Qiiiiii20s12x"
 ngshaoffset = 32
-versionformat = ">i"
+versionformat = ">I"
 
 class lazyparser(object):
     """