--- a/mercurial/scmutil.py	Sun Apr 15 19:41:34 2018 +0800
+++ b/mercurial/scmutil.py	Fri Apr 13 10:36:03 2018 -0700
@@ -436,7 +436,7 @@
 
 def resolvehexnodeidprefix(repo, prefix):
     # Uses unfiltered repo because it's faster when prefix is ambiguous/
-    # This matches the "shortest" template function.
+    # This matches the shortesthexnodeidprefix() function below.
     node = repo.unfiltered().changelog._partialmatch(prefix)
     if node is None:
         return
@@ -445,7 +445,10 @@
 
 def shortesthexnodeidprefix(repo, hexnode, minlength=1):
     """Find the shortest unambiguous prefix that matches hexnode."""
-    return repo.changelog.shortest(hexnode, minlength)
+    # _partialmatch() of filtered changelog could take O(len(repo)) time,
+    # which would be unacceptably slow. so we look for hash collision in
+    # unfiltered space, which means some hashes may be slightly longer.
+    return repo.unfiltered().changelog.shortest(hexnode, minlength)
 
 def isrevsymbol(repo, symbol):
     """Checks if a symbol exists in the repo.