help: document branch template keyword stable

help: document branch template keyword

The branch keyword was added in d79fdff55627

--- a/mercurial/help/templates.txt	Sat Feb 26 10:40:22 2011 -0600
+++ b/mercurial/help/templates.txt	Sun Feb 27 12:07:09 2011 +0100
@@ -25,6 +25,9 @@
 
 :author: String. The unmodified author of the changeset.
 
+:branch: String. The name of the branch on which the changeset was
+    committed.
+
 :branches: List of strings. The name of the branch on which the
     changeset was committed. Will be empty if the branch name was
     default.