revlog: allow duplicates

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Hash: SHA1

revlog: allow duplicates

If two branches make the same change to the same parent, the result
will be an identical hash. Git apparently does this all the time. Deal
with it gracefully.

manifest hash: c6217eab4b310e1ae529dd75ab90e717dbe5d55d
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iD8DBQFCqU61ywK+sNU5EO8RAkFqAJ9KhWUQgjZbzzB/+mTkolH0GkT1awCfa+Mj
ulbI4xCRZcvfQE492mcNwQA=
=N6In
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mercurial/revlog.py

@@ -246,6 +246,9 b' class revlog:'
 
         node = hash(text, p1, p2)
 
+        if node in self.nodemap:
+            return node
+
         n = self.count()
         t = n - 1