t/perf/p3400: speed up setup using fast-import

The setup phase in 't/perf/p3400-rebase.sh' generates 100 commits to
simulate a noisy history. It currently uses a shell loop that invokes
'git add', 'git commit', 'test_seq', and 'sort' in each iteration.
This incurs significant overhead due to repeated process spawning.

Optimize the setup by using 'git fast-import' to generate the commit
history. Additionally, pre-compute the forward and reversed file contents
to avoid repetitive execution of 'seq' and 'sort'.

To ensure the test measures rebase performance against a consistent
object layout (rather than the suboptimal pack/loose objects created
by the raw import), perform a full repack (`git repack -a -d`) at the
end of the setup.

This reduces the setup time significantly while maintaining the validity
of the subsequent performance tests.

Performance enhancement (Average value of 5 tests):
            Real        Rebase
  Before:  29.045s      13.34s
   After:  21.989s      12.84s

Measured on Lenovo Yoga 2020, Ubuntu 24.04.

Signed-off-by: Tian Yuchen <a3205153416@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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