diff -r 4c52dd406adc -r 01224c28e0ed mercurial/filemerge.py
--- a/mercurial/filemerge.py	Tue Oct 06 17:39:13 2015 -0700
+++ b/mercurial/filemerge.py	Tue Oct 06 22:54:14 2015 -0700
@@ -38,7 +38,7 @@
 # Merge tools to document.
 internalsdoc = {}
 
-def internaltool(name, trymerge, onfailure=None):
+def internaltool(name, trymerge, onfailure=None, precheck=None):
     '''return a decorator for populating internal merge tool table'''
     def decorator(func):
         fullname = ':' + name
@@ -48,6 +48,7 @@
         internalsdoc[fullname] = func
         func.trymerge = trymerge
         func.onfailure = onfailure
+        func.precheck = precheck
         return func
     return decorator