t3427: accelerate this test by using fast-export and fast-import

t3427: accelerate this test by using fast-export and fast-import

fast-export and fast-import can easily handle the simple rewrite that
was being done by filter-branch, and should be faster on systems with a
slow fork.  Measuring the overall time taken for all of t3427 (not just
the difference between filter-branch and fast-export/fast-import) shows
a speedup of about 5% on Linux and 11% on Mac.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>