t3404: make the 'rebase.missingCommitsCheck=ignore' test more focused

t3404: make the 'rebase.missingCommitsCheck=ignore' test more focused

The test 'rebase -i respects rebase.missingCommitsCheck = warn' is
mainly interested in the warning about the dropped commits, but it
checks the whole output of 'git rebase', including progress lines and
what not that are not at all relevant to 'rebase.missingCommitsCheck',
but make it necessary to update this test whenever e.g. the way we
show progress is updated (as it will happen in one of the later
patches of this series).

Modify the test to verify only the first four lines of 'git rebase's
output that contain all the important lines, notably the line
containing the "Warning:" itself and the oneline log of the dropped
commit.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>