tree-diff: move all action-taking code out of compare_tree_entry()

tree-diff: move all action-taking code out of compare_tree_entry()

- let it do only comparison.

This way the code is cleaner and more structured - cmp function only
compares, and the driver takes action based on comparison result.

There should be no change in performance, as effectively, we just move
if series from on place into another, and merge it to was-already-there
same switch/if, so the result is maybe a little bit faster.

Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
Signed-off-by: Junio C Hamano <gitster@pobox.com>