diff -r fdc6eb1d650d -r ffd574c144d2 mercurial/ui.py
--- a/mercurial/ui.py	Sun Nov 11 17:34:46 2018 +0900
+++ b/mercurial/ui.py	Sun Nov 11 19:35:33 2018 +0900
@@ -1724,14 +1724,14 @@
         """
         self._loggers[name] = logger
 
-    def log(self, event, *msg, **opts):
+    def log(self, event, msgfmt, *msgargs, **opts):
         '''hook for logging facility extensions
 
         event should be a readily-identifiable subsystem, which will
         allow filtering.
 
-        *msg should be a newline-terminated format string to log, and
-        then any values to %-format into that format string.
+        msgfmt should be a newline-terminated format string to log, and
+        *msgargs are %-formatted into it.
 
         **opts currently has no defined meanings.
         '''
@@ -1741,6 +1741,7 @@
                          if l.tracked(event)]
         if not activeloggers:
             return
+        msg = msgfmt % msgargs
         # guard against recursion from e.g. ui.debug()
         registeredloggers = self._loggers
         self._loggers = {}