t/t7704-repack-cruft.sh: avoid failures during long-running tests

t/t7704-repack-cruft.sh: avoid failures during long-running tests

On systems where running t7704.09 takes longer than 10 seconds, the test
can fail.

The test works by doing the following:

  - First write three unreachable objects, backdating the mtime for a
    single object ($foo) which we expect to prune.

  - Repack the repository into a pack containing reachable objects, and
    another three cruft packs, each containing one of the objects
    written in the previous step.

  - Backdate the mtimes of the cruft pack *.mtimes files themselves.
    (Note that this does not affect what is pruned further down in the
    test, but is done to ensure that the cruft packs are rewritten
    during that step).

  - Then repack with --cruft-expiration=10.seconds.ago, expecting to
    prune one of the three unreachable objects written in the first
    step.

  - Assert that the surviving cruft packs were rewritten, object $foo is
    pruned, and unreachable objects $bar, and $baz remain in the
    repository.

If longer than 10 seconds pass between writing the three unreachable
objects (the first step) and the "git repack --cruft" (the fourth step),
we will mistakenly prune more objects than expected, causing the test to
fail.

The $foo object which we expect to prune has its mtime set back to
10,000 seconds relative to the current time, but we prune it with a
cutoff of 10.seconds.ago.

Instead, set the cutoff to be 1,000 seconds to give the test much longer
time to run without failing. This helps platforms where running
individual tests can perform slowly, on my machine this test runs much
more quickly:

    $ hyperfine './t7704-repack-cruft.sh --run=9'
    Benchmark 1: ./t7704-repack-cruft.sh --run=9
      Time (mean ± σ):     647.4 ms ±  30.7 ms    [User: 528.5 ms, System: 124.1 ms]
      Range (min … max):   594.1 ms … 696.5 ms    10 runs

Reported-by: Randall Becker <randall.becker@nexbridge.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>