repository: fix free problem with repo_clear(the_repository)

the_repository is special. One of the special things about it is that
it does not allocate a new index_state object like submodules but
points to the global the_index variable instead. As a global variable,
the_index cannot be free()'d.

Add an exception for this in repo_clear(). In the future perhaps we
would be able to allocate the_repository's index on heap too. Then we
can revert this.

the_repository->index remains pointed to a clean the_index even after
repo_clear() so that it could still be used next time (e.g. in a crazy
use case where a dev switches repo in the same process).

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

repository.c

@@ -220,7 +220,8 @@ void repo_clear(struct repository *repo)
 
 	if (repo->index) {
 		discard_index(repo->index);
-		FREE_AND_NULL(repo->index);
+		if (repo->index != &the_index)
+			FREE_AND_NULL(repo->index);
 	}
 }