config.txt: move difftool.* to a separate file

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

Documentation/config.txt

@@ -321,20 +321,7 @@ include::config/completion.txt[]
 
 include::config/diff.txt[]
 
-difftool.<tool>.path::
-	Override the path for the given tool.  This is useful in case
-	your tool is not in the PATH.
-
-difftool.<tool>.cmd::
-	Specify the command to invoke the specified diff tool.
-	The specified command is evaluated in shell with the following
-	variables available:  'LOCAL' is set to the name of the temporary
-	file containing the contents of the diff pre-image and 'REMOTE'
-	is set to the name of the temporary file containing the contents
-	of the diff post-image.
-
-difftool.prompt::
-	Prompt before each invocation of the diff tool.
+include::config/difftool.txt[]
 
 fastimport.unpackLimit::
 	If the number of objects imported by linkgit:git-fast-import[1]

Documentation/config/difftool.txt

@@ -0,0 +1,14 @@
+difftool.<tool>.path::
+	Override the path for the given tool.  This is useful in case
+	your tool is not in the PATH.
+
+difftool.<tool>.cmd::
+	Specify the command to invoke the specified diff tool.
+	The specified command is evaluated in shell with the following
+	variables available:  'LOCAL' is set to the name of the temporary
+	file containing the contents of the diff pre-image and 'REMOTE'
+	is set to the name of the temporary file containing the contents
+	of the diff post-image.
+
+difftool.prompt::
+	Prompt before each invocation of the diff tool.