t5520 (pull): update test description

d09e79c (git-pull: allow pulling into an empty repository, 2006-11-16)
created the file with the description.  At the time, there were only
tests to check pulling into an empty repository.  The description was
never updated even after more tests, unrelated to the original, were
added.  Fix this now.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
t5520 (pull): use test_config where appropriate

Configuration from test_config does not last beyond the end of the
current test assertion, making each test easier to think about in
isolation.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>