t1507: run commands within test_expect_success

t1507: run commands within test_expect_success

The expected test style is to have all commands tested within a
test_expect_success block. Move the generation of the 'expect' text into
their corresponding blocks. While we're at it, insert a second
`commit=$(git rev-parse HEAD)` into the next test case so that it's
clear where $commit is coming from.

The biggest advantage of doing this is that we now check the return code
of `git rev-parse HEAD` so we can catch it in case it fails.

This patch is best viewed with `--color-moved --ignore-all-space`.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>