t7503: verify proper hook execution

t7503: verify proper hook execution

t7503 did not verify that the expected hooks actually ran during
testing. Fix that by making the hook scripts write their $0 into a file
so that we can compare actual execution vs. expected execution.

While we're at it, do some test style cleanups, such as using
write_script() and doing setup inside a test_expect_success block.

Improved-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>