fix hg diff -r ''

mercurial/cmdutil.py

@@ -18,7 +18,7 b' def revpair(repo, revs):'
     be None, meaning use working dir.'''
 
     def revfix(repo, val, defval):
-        if not val and val != 0:
+        if not val and val != 0 and defval is not None:
             val = defval
         return repo.lookup(val)
 

tests/test-diffdir

@@ -13,3 +13,8 b' hg diff --nodates -r tip'
 
 echo foo > a
 hg diff --nodates
+
+hg diff -r ""
+hg diff -r tip -r ""
+
+true

tests/test-diffdir.out

@@ -18,3 +18,5 b' diff -r acd8075edac9 b'
 +++ b/b
 @@ -0,0 +1,1 @@
 +123
+abort: Ambiguous identifier!
+abort: Ambiguous identifier!