t1507: stop losing return codes of git commands

t1507: stop losing return codes of git commands

The return code of git commands are lost when a command is in a
non-assignment command substitution in favour of the surrounding
command's. Rewrite instances of this so that git commands run
on their own.

In commit_subject(), use a `tformat` instead of `format` since,
previously, we were testing the output of a command substitution which
didn't care if there was a trailing newline since it was automatically
stripped. Since we use test_cmp() now, the trailing newline matters so
use `tformat` to always output it.

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