diff -r d30a19d10441 -r 943248e47864 mercurial/cmdutil.py
--- a/mercurial/cmdutil.py	Sun Oct 14 11:16:22 2018 -0400
+++ b/mercurial/cmdutil.py	Wed Oct 17 21:00:36 2018 +0800
@@ -149,7 +149,7 @@
 ]
 
 mergetoolopts = [
-    ('t', 'tool', '', _('specify merge tool')),
+    ('t', 'tool', '', _('specify merge tool'), _('TOOL')),
 ]
 
 similarityopts = [