t34xx: don't expire reflogs where it matters

We have a couple of tests in the t34xx range that rely on reflogs. This
never really used to be a problem, but in a subsequent commit we will
change the default maintenance strategy from "gc" to "geometric", and
this will cause us to drop all reflogs in these tests.

This may seem surprising and like a bug at first, but it's actually not.
The main difference between these two strategies is that the "gc"
strategy will skip all maintenance in case the object database is in a
well-optimized state. The "geometric" strategy has separate subtasks
though, and the conditions for each of these tasks is evaluated on a
case by case basis. This means that even if the object database is in
good shape, we may still decide to expire reflogs.

So why is that a problem? The issue is that Git's test suite hardcodes
the committer and author dates to a date in 2005. Interestingly though,
these hardcoded dates not only impact the commits, but also the reflog
entries. The consequence is that all newly written reflog entries are
immediately considered stale as our reflog expiration threshold is in
the range of weeks, only. It follows that executing `git reflog expire`
will thus immediately purge all reflog entries.

This hasn't been a problem in our test suite by pure chance, as the
repository shapes simply didn't cause us to perform actual garbage
collection. But with the upcoming "geometric" strategy we _will_ start
to execute `git reflog expire`, thus surfacing this issue.

Prepare for this by explicitly disabling reflog expiration in tests
impacted by this upcoming change.

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

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