t3404.6 breaks on master under GIT_FSMONITOR_TEST

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

The GIT_FSMONITOR_TEST variable allows you to roundtrip the fsmonitor
codpath in the whole test suite. On both Debian & CentOS this breaks for
me:

    (cd t && GIT_FSMONITOR_TEST=$PWD/t7519/fsmonitor-all ./t3404-rebase-interactive.sh -i)

Whereas this works:

    (cd t && GIT_FSMONITOR_TEST=$PWD/t7519/fsmonitor-all GIT_SKIP_TESTS=t3404.6 ./t3404-rebase-interactive.sh -i)

The entirety of the rest of the test suite still passes with
GIT_FSMONITOR_TEST.

This has been failing ever since GIT_FSMONITOR_TEST was introduced in
883e248b8a ("fsmonitor: teach git to optionally utilize a file system
monitor to speed up detecting new or changed files.", 2017-09-22). Under
-v -x -i:

    + echo test_must_fail: command succeeded: env FAKE_LINES=exec_echo_foo_>file1 1 git rebase -i HEAD^
    test_must_fail: command succeeded: env FAKE_LINES=exec_echo_foo_>file1 1 git rebase -i HEAD^
    + return 1
    error: last command exited with $?=1
    not ok 6 - rebase -i with the exec command checks tree cleanness
    #
    #               git checkout master &&
    #               set_fake_editor &&
    #               test_must_fail env FAKE_LINES="exec_echo_foo_>file1 1" git rebase -i HEAD^ &&

Maybe once this is fixed running the test suite under GIT_FSMONITOR_TEST
would be a useful Travis target, but I don't know the current status of
adding new options to Travis.