t1300: add tests for missing keys

t1300: add tests for missing keys

There are several tests in t1300-config.sh that validate failing
invocations of "git config".  However, there are no tests that check
what happens when "git config" is asked to retrieve a value for a
missing key.

Add tests that check this for various combinations of "<section>.<key>"
and "<section>.<subsection>.<key>".

Signed-off-by: Andrei Rybak <rybak.a.v@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>