t5541-http-push: make grep expression check for one line only

t5541-http-push: make grep expression check for one line only

Don't feed a multiple-line pattern to grep and expect the them to match
with lines in order.

Simplify the grep expressions in the non-fast-forward tests to check
only for the first line of the non-fast-forward warning - having that
line should be enough assurance that the full warning is printed.

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>