revset: allow rev(-1) to indicate null revision (BC) This can simplify the conversion from numeric revision to string. Without it, we have to handle -1 specially because repo['-1'] != repo[-1]. The -1 revision is not officially documented, but this change makes sense assuming that "rev(%d)" exists for scripting or third-party tools.

--- a/tests/test-revset.t	Fri Jan 23 20:30:49 2015 -0800
+++ b/tests/test-revset.t	Sat Jan 10 12:56:38 2015 +0900
@@ -465,7 +465,9 @@
   9
 
 Test explicit numeric revision
+  $ log 'rev(-2)'
   $ log 'rev(-1)'
+  -1
   $ log 'rev(0)'
   0
   $ log 'rev(9)'