[PATCH 4 of 7 STABLE] help: refer ":merge3" instead of "internal:merge3"

[FUJIWARA Katsunori]

# HG changeset patch
# User FUJIWARA Katsunori <foozy at lares.dti.ne.jp>
# Date 1414777388 -32400
#      Sat Nov 01 02:43:08 2014 +0900
# Branch stable
# Node ID 0dae9147c3b6bd1a854734cdadc13910e6b9b85e
# Parent  692bde7f486ddce18264c10fd4c7857b6acb5fb3
help: refer ":merge3" instead of "internal:merge3"

According to warning message (introduced by 38e0363dcbe0) in
filemerge.py, the former should be used as official name.

diff --git a/mercurial/help/config.txt b/mercurial/help/config.txt
--- a/mercurial/help/config.txt
+++ b/mercurial/help/config.txt
@@ -982,7 +982,8 @@
   launching external tool.  Options are ``true``, ``false``, ``keep`` or
   ``keep-merge3``. The ``keep`` option will leave markers in the file if the
   premerge fails. The ``keep-merge3`` will do the same but include information
-  about the base of the merge in the marker (see internal:merge3).
+  about the base of the merge in the marker (see internal :merge3 in
+  :hg:`help merge-tools`).
   Default: True
 
 ``binary``