hg-to-git: avoid raising a string exception

This fixes the following warning:
hg-to-git.py:92: DeprecationWarning: raising a string exception is deprecated

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/hg-to-git/hg-to-git.py

@@ -89,7 +89,7 @@ try:
         if o in ('-v', '--verbose'):
             verbose = True
     if len(args) != 1:
-        raise('params')
+        raise Exception('params')
 except:
     usage()
     sys.exit(1)