util: allow sha1() with no args stable

util: allow sha1() with no args

Normally this works because we replace util.sha1 with hashlib.sha1
after first use, but if the first user doesn't provide an arg, it
breaks.

--- a/mercurial/util.py	Mon Oct 31 14:22:11 2011 -0500
+++ b/mercurial/util.py	Mon Oct 31 14:22:11 2011 -0500
@@ -73,14 +73,14 @@
 
 # Python compatibility
 
-def sha1(s):
+def sha1(s=''):
     return _fastsha1(s)
 
 _notset = object()
 def safehasattr(thing, attr):
     return getattr(thing, attr, _notset) is not _notset
 
-def _fastsha1(s):
+def _fastsha1(s=''):
     # This function will import sha1 from hashlib or sha (whichever is
     # available) and overwrite itself with it on the first call.
     # Subsequent calls will go directly to the imported function.