t9350: redirect input to only fast-import

t9350: redirect input to only fast-import

A number of tests in "t9350-fast-export.sh" are using sub-shells to
redirect content to a number of commands instead of only
`git fast-import`.

This is confusing and possibly error-prone, so let's change those tests
so that no sub-shell is used and the content goes only to
`git fast-import`.

Reported-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>