t9100: stop depending on commit timestamps

t9100: stop depending on commit timestamps

An earlier "fix" to this script gave up updating it not to rely on
the current time because we cannot control what timestamp subversion
gives its commits.  We however could solve the issue in a different
way and still use deterministic timestamps on Git commits.

One fix would be to sort the list of trees before removing duplicates,
but that loses information:

  - we do care that the fetched history is in the same order

  - there's a tree which appears twice in the history, and we'd want to
    make sure that it's there both times

So instead, let's de-duplicate using a hash (preserving the order), and
drop only lines with identical trees and subjects (preserving the tree
which appears twice, since it has different subjects each time).

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>