t7800: modernize tests

Eliminate a lot of redundant work by using test_config().
Catch more return codes by more use of temporary files
and test_cmp.

The original tests relied upon restore_test_defaults()
from the previous test to provide the next test with a sane
environment.  Make the tests do their own setup so that they
are not dependent on the success of the previous test.
The end result is shorter tests and better test isolation.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>