midx: enable core.multiPackIndex by default

The core.multiPackIndex setting has been around since c4d25228ebb
(config: create core.multiPackIndex setting, 2018-07-12), but has been
disabled by default. If a user wishes to use the multi-pack-index
feature, then they must enable this config and run 'git multi-pack-index
write'.

The multi-pack-index feature is relatively stable now, so make the
config option true by default. For users that do not use a
multi-pack-index, the only extra cost will be a file lookup to see if a
multi-pack-index file exists (once per process, per object directory).

Also, this config option will be referenced by an upcoming
"incremental-repack" task in the maintenance builtin, so move the config
option into the repository settings struct. Note that if
GIT_TEST_MULTI_PACK_INDEX=1, then we want to ignore the config option
and treat core.multiPackIndex as enabled.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/repo-settings.c b/repo-settings.c
index 0918408..5bd2c22 100644
--- a/repo-settings.c
+++ b/repo-settings.c
@@ -1,6 +1,7 @@
 #include "cache.h"
 #include "config.h"
 #include "repository.h"
+#include "midx.h"
 
 #define UPDATE_DEFAULT_BOOL(s,v) do { if (s == -1) { s = v; } } while(0)
 
@@ -47,6 +48,11 @@ void prepare_repo_settings(struct repository *r)
 		r->settings.pack_use_sparse = value;
 	UPDATE_DEFAULT_BOOL(r->settings.pack_use_sparse, 1);
 
+	value = git_env_bool(GIT_TEST_MULTI_PACK_INDEX, 0);
+	if (value || !repo_config_get_bool(r, "core.multipackindex", &value))
+		r->settings.core_multi_pack_index = value;
+	UPDATE_DEFAULT_BOOL(r->settings.core_multi_pack_index, 1);
+
 	if (!repo_config_get_bool(r, "feature.manyfiles", &value) && value) {
 		UPDATE_DEFAULT_BOOL(r->settings.index_version, 4);
 		UPDATE_DEFAULT_BOOL(r->settings.core_untracked_cache, UNTRACKED_CACHE_WRITE);