commit-reach: guard !FIND_ALL early exit with generation ordering check

commit-reach: guard !FIND_ALL early exit with generation ordering check

When paint_down_to_common() falls back to commit-date ordering (for
v1 commit graphs without corrected commit dates), the !FIND_ALL early
exit incorrectly fires.  The exit assumes the queue is generation-
ordered, so the first RESULT commit found must be the shallowest.
With date ordering this is not guaranteed: a closer merge base with
a lower committer date (clock skew) may still be in the queue behind
deeper commits.

Add a gen_ordered flag that is cleared when the date fallback fires,
and require it for the early exit.

Update the test from the previous commit to test_expect_success.

Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>