config: move config_from_gitmodules to submodule-config.c

The .gitmodules file is not meant as a place to store arbitrary
configuration to distribute with the repository.

Move config_from_gitmodules() out of config.c and into
submodule-config.c to make it even clearer that it is not a mechanism to
retrieve arbitrary configuration from the .gitmodules file.

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Acked-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule-config.h

@@ -2,6 +2,7 @@
 #define SUBMODULE_CONFIG_CACHE_H
 
 #include "cache.h"
+#include "config.h"
 #include "hashmap.h"
 #include "submodule.h"
 #include "strbuf.h"
@@ -55,4 +56,14 @@ void submodule_free(struct repository *r);
  */
 int check_submodule_name(const char *name);
 
+/*
+ * Note: This function exists solely to maintain backward compatibility with
+ * 'fetch' and 'update_clone' storing configuration in '.gitmodules' and should
+ * NOT be used anywhere else.
+ *
+ * Runs the provided config function on the '.gitmodules' file found in the
+ * working directory.
+ */
+extern void config_from_gitmodules(config_fn_t fn, void *data);
+
 #endif /* SUBMODULE_CONFIG_H */