hook tests: test for exact "pre-push" hook input

hook tests: test for exact "pre-push" hook input

Extend the tests added in ec55559f937 (push: Add support for pre-push
hooks, 2013-01-13) to exhaustively test for the exact input we're
expecting. This ensures that we e.g. don't miss a trailing newline.

Appending to a file called "actual" is the established convention in
this test for hooks, see the rest of the tests added in
ec55559f937 (push: Add support for pre-push hooks, 2013-01-13). Let's
follow that convention here.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>