sequencer: extract helper to update active_cache_tree

sequencer: extract helper to update active_cache_tree

This patch extracts the code from is_index_unchanged() to initialize or
update the index' cache tree (i.e. a tree object reflecting the current
index' top-level tree).

The new helper will be used in the upcoming code to support `git rebase
-i --root` via the sequencer.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>