diff -r f9c91c638378 -r 755bf1bbe0a9 mercurial/filemerge.py
--- a/mercurial/filemerge.py	Sun Jul 06 02:56:41 2014 +0900
+++ b/mercurial/filemerge.py	Sun Jul 06 02:56:41 2014 +0900
@@ -287,6 +287,9 @@
     label = ('%s:' % label).ljust(pad + 1)
     mark = '%s %s' % (label, templater.stringify(templateresult))
 
+    if mark:
+        mark = mark.splitlines()[0] # split for safety
+
     # The <<< marks add 8 to the length, and '...' adds three, so max
     # length of the actual marker is 69.
     maxlength = 80 - 8 - 3