t1507: inline full_name()

t1507: inline full_name()

Before, we were running `test_must_fail full_name`. However,
`test_must_fail` should only be used on git commands. Inline full_name()
so that we can use test_must_fail on the git command directly.

When full_name() was introduced in 28fb84382b (Introduce
<branch>@{upstream} notation, 2009-09-10), the `git -C` option wasn't
available yet (since it was introduced in 44e1e4d67d (git: run in a
directory given with -C option, 2013-09-09)). As a result, the helper
function removed the need to manually cd each time. However, since
`git -C` is available now, we can just use that instead and inline
full_name().

An alternate approach was taken where we taught full_name() to accept an
optional `!` arg to trigger test_must_fail behavior. However, this added
more unnecessary complexity than inlining so we inline instead.

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