config.txt: move completion.* 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

@@ -317,13 +317,7 @@ include::config/commit.txt[]
 
 include::config/credential.txt[]
 
-completion.commands::
-	This is only used by git-completion.bash to add or remove
-	commands from the list of completed commands. Normally only
-	porcelain commands and a few select others are completed. You
-	can add more commands, separated by space, in this
-	variable. Prefixing the command with '-' will remove it from
-	the existing list.
+include::config/completion.txt[]
 
 include::diff-config.txt[]
 

Documentation/config/completion.txt

@@ -0,0 +1,7 @@
+completion.commands::
+	This is only used by git-completion.bash to add or remove
+	commands from the list of completed commands. Normally only
+	porcelain commands and a few select others are completed. You
+	can add more commands, separated by space, in this
+	variable. Prefixing the command with '-' will remove it from
+	the existing list.