util: stop overwriting sha1, overwrite _fastsha1 instead

Some modules (like revlog) would import util.sha1 as _sha1. This
defeats the purpose of having util.sha1 overwrite itself with a faster
version -- revlog would end up always calling the slow version. By
always delegating to util._fastsha1 we avoid this at the cost of an
extra (but unconditional) indirection.

mercurial/util.py

@@ -21,13 +21,18 b' import imp'
 # Python compatibility
 
 def sha1(s):
+    return _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.
     try:
-        import hashlib
-        _sha1 = hashlib.sha1
+        from hashlib import sha1 as _sha1
     except ImportError:
         from sha import sha as _sha1
-    global sha1
-    sha1 = _sha1
+    global _fastsha1
+    _fastsha1 = _sha1
     return _sha1(s)
 
 import subprocess