revlog: faster hash computation when one of the parent node is null

Because we often compute sha1(nullid), it's interesting to copy a precomputed
hash of nullid instead of computing everytime the same hash. Similarly, when
one of the parents is null, we can avoid a < comparison (sort).

Overall, this change adds a string equality comparison on each hash() call,
but when p2 is null, we drop one string < comparison, and copy a hash instead
of computing it. Since it is common to have revisions with only one parent,
this change makes hash() 25% faster when cloning a big repository.

mercurial/revlog.py

@@ -42,6 +42,8 b' def gettype(q):'
 def offset_type(offset, type):
     return long(long(offset) << 16 | type)
 
+nullhash = _sha(nullid)
+
 def hash(text, p1, p2):
     """generate a hash from the given text and its parent hashes
 
@@ -49,10 +51,17 b' def hash(text, p1, p2):'
     in a manner that makes it easy to distinguish nodes with the same
     content in the revision graph.
     """
-    l = [p1, p2]
-    l.sort()
-    s = _sha(l[0])
-    s.update(l[1])
+    # As of now, if one of the parent node is null, p2 is null
+    if p2 == nullid:
+        # deep copy of a hash is faster than creating one
+        s = nullhash.copy()
+        s.update(p1)
+    else:
+        # none of the parent nodes are nullid
+        l = [p1, p2]
+        l.sort()
+        s = _sha(l[0])
+        s.update(l[1])
     s.update(text)
     return s.digest()