submodules: add RECURSE_SUBMODULES_ONLY value

Add the `RECURSE_SUBMODULES_ONLY` enum value to submodule.h.  This enum
value will be used in a later patch to push to indicate that only
submodules should be pushed, while the superproject should remain
unpushed.

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

submodule-config.c

@@ -251,6 +251,8 @@ static int parse_push_recurse(const char *opt, const char *arg,
 			return RECURSE_SUBMODULES_ON_DEMAND;
 		else if (!strcmp(arg, "check"))
 			return RECURSE_SUBMODULES_CHECK;
+		else if (!strcmp(arg, "only"))
+			return RECURSE_SUBMODULES_ONLY;
 		else if (die_on_error)
 			die("bad %s argument: %s", opt, arg);
 		else

submodule.h

@@ -6,6 +6,7 @@ struct argv_array;
 struct sha1_array;
 
 enum {
+	RECURSE_SUBMODULES_ONLY = -5,
 	RECURSE_SUBMODULES_CHECK = -4,
 	RECURSE_SUBMODULES_ERROR = -3,
 	RECURSE_SUBMODULES_NONE = -2,