read-cache.c: kill read_index()

read_index() shares the same problem as hold_locked_index(): it
assumes $GIT_DIR/index. Move all call sites to repo_read_index()
instead. read_index_preload() and read_index_unmerged() are also
killed as a consequence.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/commit.c b/builtin/commit.c
index e29fb5e..19eb6cf 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1367,7 +1367,7 @@ int cmd_status(int argc, const char **argv, const char *prefix)
 	if (status_format != STATUS_FORMAT_PORCELAIN &&
 	    status_format != STATUS_FORMAT_PORCELAIN_V2)
 		progress_flag = REFRESH_PROGRESS;
-	read_index(&the_index);
+	repo_read_index(the_repository);
 	refresh_index(&the_index,
 		      REFRESH_QUIET|REFRESH_UNMERGED|progress_flag,
 		      &s.pathspec, NULL, NULL);