config.txt: move diff-config.txt to config/

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

Documentation/config.txt

@@ -319,7 +319,7 @@ include::config/credential.txt[]
 
 include::config/completion.txt[]
 
-include::diff-config.txt[]
+include::config/diff.txt[]
 
 difftool.<tool>.path::
 	Override the path for the given tool.  This is useful in case

Documentation/config/diff.txt

@@ -177,7 +177,7 @@ diff.tool::
 	Any other value is treated as a custom diff tool and requires
 	that a corresponding difftool.<tool>.cmd variable is defined.
 
-include::mergetools-diff.txt[]
+include::../mergetools-diff.txt[]
 
 diff.indentHeuristic::
 	Set this option to `true` to enable experimental heuristics