split-index: fix memory leak in `move_cache_to_base_index()`

split-index: fix memory leak in `move_cache_to_base_index()`

In `move_cache_to_base_index()` we move the index cache of the main
index into the split index, which is used when writing a shared index.
But we don't release the old split index base in case we already had a
split index before this operation, which can thus leak memory.

Plug the leak by releasing the previous base.

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