--- a/tests/test-template-functions.t	Sat Jun 16 13:17:11 2018 +0900
+++ b/tests/test-template-functions.t	Sat Jun 16 14:10:12 2018 +0900
@@ -216,6 +216,32 @@
   $ hg debugtemplate '{"foo/bar"|stripdir}|{"foo/"|stripdir}|{"foo"|stripdir}|\n'
   foo|foo|foo|
 
+commondir() filter:
+
+  $ hg debugtemplate '{""|splitlines|commondir}\n'
+  
+  $ hg debugtemplate '{"foo/bar\nfoo/baz\nfoo/foobar\n"|splitlines|commondir}\n'
+  foo
+  $ hg debugtemplate '{"foo/bar\nfoo/bar\n"|splitlines|commondir}\n'
+  foo
+  $ hg debugtemplate '{"/foo/bar\n/foo/bar\n"|splitlines|commondir}\n'
+  foo
+  $ hg debugtemplate '{"/foo\n/foo\n"|splitlines|commondir}\n'
+  
+  $ hg debugtemplate '{"foo/bar\nbar/baz"|splitlines|commondir}\n'
+  
+  $ hg debugtemplate '{"foo/bar\nbar/baz\nbar/foo\n"|splitlines|commondir}\n'
+  
+  $ hg debugtemplate '{"foo/../bar\nfoo/bar"|splitlines|commondir}\n'
+  foo
+  $ hg debugtemplate '{"foo\n/foo"|splitlines|commondir}\n'
+  
+
+  $ hg log -r null -T '{rev|commondir}'
+  hg: parse error: argument is not a list of text
+  (template filter 'commondir' is not compatible with keyword 'rev')
+  [255]
+
 Add a dummy commit to make up for the instability of the above:
 
   $ echo a > a