commit: don't rewrite shared index unnecessarily

commit: don't rewrite shared index unnecessarily

Remove a cache invalidation which would cause the shared index to be
rewritten on as-is commits.

When the cache-tree has changed, we need to update it.  But we don't
necessarily need to update the shared index.  So setting
active_cache_changed to SOMETHING_CHANGED is unnecessary.  Instead, we
let update_main_cache_tree just update the CACHE_TREE_CHANGED bit.

In order to test this, make test-dump-split-index not segfault on
missing replace_bitmap/delete_bitmap.  This new codepath is not called
now that the test passes, but is necessary to avoid a segfault when the
new test is run with the old builtin/commit.c code.

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