checkout branch: prime cache-tree fully

checkout branch: prime cache-tree fully

When switching to another branch, the earlier code relied on incremental
invalidation of cache-tree entries to degrade it.  While it is not wrong
per-se, we know that the resulting index must fully match the branch we
are switching to unless the -m (merge) option is used.

We should simply fully re-prime the cache-tree using the new tree object
in such a case.  And for safety, invalidate the cache-tree as a whole in
other cases.

Signed-off-by: Junio C Hamano <gitster@pobox.com>