diff: futureproof "stop feeding the backend early" logic

diff: futureproof "stop feeding the backend early" logic

Refactor the "do not stop feeding the backend early" logic into a small
helper function and use it in both run_diff_files() and diff_tree() that
has the stop-early optimization. We may later add other types of diffcore
transformation that require to look at the whole result like diff-filter
does, and having the logic in a single place is essential for longer term
maintainability.

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