submodule.c: use GIT_DIR_ENVIRONMENT consistently

In C code we have the luxury of having constants for all the important
things that are hard coded. This is the only place in C that hard codes
the git directory environment variable, so fix it.

Signed-off-by: Stefan Beller <sbeller@google.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule.c

@@ -1333,5 +1333,6 @@ void prepare_submodule_repo_env(struct argv_array *out)
 		if (strcmp(*var, CONFIG_DATA_ENVIRONMENT))
 			argv_array_push(out, *var);
 	}
-	argv_array_push(out, "GIT_DIR=.git");
+	argv_array_pushf(out, "%s=%s", GIT_DIR_ENVIRONMENT,
+			 DEFAULT_GIT_DIR_ENVIRONMENT);
 }