wt-status: provide function to expose status for trees

wt-status: provide function to expose status for trees

The "wt-status" subsystem is responsible for printing status information
around the current state of the working tree. This most importantly
includes information around whether the working tree or the index have
any changes.

We're about to introduce a new command where the changes in neither of
them are actually relevant to us. Instead, what we want is to format the
changes between two different trees. While it is a little bit of a
stretch to add this as functionality to _working tree_ status, it
doesn't make any sense to open-code this functionality, either.

Implement a new function `wt_status_collect_changes_trees()` that diffs
two trees and formats the status accordingly. This function is not yet
used, but will be in a subsequent commit.

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