config.txt: move fmt-merge-msg-config.txt to config/

Note that this file is not directly included in config.txt but through
merge-config.txt and it's in "merge" section instead of a separate
"fmtMergeMsg" section like others.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fmt-merge-msg.txt

@@ -51,7 +51,7 @@ OPTIONS
 
 CONFIGURATION
 -------------
-include::fmt-merge-msg-config.txt[]
+include::config/fmt-merge-msg.txt[]
 
 merge.summary::
 	Synonym to `merge.log`; this is deprecated and will be removed in

Documentation/merge-config.txt

@@ -30,7 +30,7 @@ merge.verifySignatures::
 	If true, this is equivalent to the --verify-signatures command
 	line option. See linkgit:git-merge[1] for details.
 
-include::fmt-merge-msg-config.txt[]
+include::config/fmt-merge-msg.txt[]
 
 merge.renameLimit::
 	The number of files to consider when performing rename detection