repo-settings: track defaults close to struct repo_settings

The default values for `struct repo_settings` are set up in
`prepare_repo_settings()`. This is somewhat different from how we
typically do this, namely by providing an `INIT` macro that sets up the
default values for us.

Refactor the code to do the same.

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

repo-settings.h

@@ -50,6 +50,11 @@ struct repo_settings {
 
 	int core_multi_pack_index;
 };
+#define REPO_SETTINGS_INIT { \
+	.index_version = -1, \
+	.core_untracked_cache = UNTRACKED_CACHE_KEEP, \
+	.fetch_negotiation_algorithm = FETCH_NEGOTIATION_CONSECUTIVE, \
+}
 
 void prepare_repo_settings(struct repository *r);