revlog: ensure that flags do not overflow 2 bytes

This patch adds a line that ensures we are not setting by mistake a set of flags
overlfowing the 2 bytes they are allocated. Given the way the data is packed in
the revlog header, overflowing 2 bytes will result in setting a wrong offset.

mercurial/revlog.py

@@ -72,6 +72,8 b' def gettype(q):'
     return int(q & 0xFFFF)
 
 def offset_type(offset, type):
+    if (type & ~REVIDX_KNOWN_FLAGS) != 0:
+        raise ValueError('unknown revlog index flags')
     return long(long(offset) << 16 | type)
 
 _nullhash = hashlib.sha1(nullid)