reset: stop assuming that the caller passes in a clean index

In 652bd0211d (rebase: use 'skip_cache_tree_update' option, 2022-11-10), we updated `reset_working_tree()` to stop updating the index tree cache. This was done as a performance optimization: the function is only called by "sequencer.c" and "rebase.c", both of which assume a clean index before they perform their operation, so we know that the end result will be a clean index, too. Consequently, we can skip recomputing the cache as we can instead use `prime_cache_tree()` directly. In a subsequent commit we're about to add a new caller though where the assumption doesn't hold anymore: the index may be dirty before calling `reset_working_tree()`, and consequently we cannot prime the cache with a given tree anymore as the index and tree will mismatch. Adapt the logic so that we only skip the cache tree update in case we're doing a hard reset. While we could introduce logic that only skips the update in case the incoming index was dirty already, that doesn't really feel worth it: after all, the mentioned commit says itself that the performance improvement was negligible anyway. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>