diff -r 794afa91f0a5 -r df0873ab5c14 mercurial/error.py
--- a/mercurial/error.py	Thu Aug 02 22:44:41 2018 +0300
+++ b/mercurial/error.py	Fri Apr 13 23:37:53 2018 -0700
@@ -58,6 +58,9 @@
     def __str__(self):
         return RevlogError.__str__(self)
 
+class AmbiguousPrefixLookupError(LookupError):
+    pass
+
 class FilteredLookupError(LookupError):
     pass