make revlog.LookupError inherit from KeyError

This will improve integration with external applications which depend on
KeyError, which is usually raised by __getitem__ calls.

mercurial/revlog.py

@@ -32,7 +32,7 b' REVLOG_DEFAULT_VERSION = REVLOG_DEFAULT_'
 class RevlogError(Exception):
     pass
 
-class LookupError(RevlogError):
+class LookupError(RevlogError, KeyError):
     def __init__(self, name, index, message):
         self.name = name
         if isinstance(name, str) and len(name) == 20: