submodule-config: move submodule-config functions to submodule-config.c

Migrate the functions used to initialize the submodule-config to
submodule-config.c so that the callback routine used in the
initialization process can be static and prevent it from being used
outside of initializing the submodule-config through the main API.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/ls-files.c

@@ -19,6 +19,7 @@
 #include "pathspec.h"
 #include "run-command.h"
 #include "submodule.h"
+#include "submodule-config.h"
 
 static int abbrev;
 static int show_deleted;