config: make dependency on repo in read_early_config() explicit

The `read_early_config()` function can be used to read configuration
where a repository has not yet been set up. As such, it is optional
whether or not `the_repository` has already been initialized. If it was
initialized we use its commondir and gitdir. If not, the function will
try to detect the Git directories by itself and, if found, also parse
their config files.

This means that we implicitly rely on `the_repository`. Make this
dependency explicit by passing a `struct repository`. This allows us to
again drop the `USE_THE_REPOSITORY_VARIABLE` define in "config.c".

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-config.c

@@ -96,7 +96,8 @@ int cmd__config(int argc, const char **argv)
 	struct config_set cs;
 
 	if (argc == 3 && !strcmp(argv[1], "read_early_config")) {
-		read_early_config(early_config_cb, (void *)argv[2]);
+		read_early_config(the_repository, early_config_cb,
+				  (void *)argv[2]);
 		return 0;
 	}