t4034: use test_config/test_unconfig to set/unset git config variables

t4034: use test_config/test_unconfig to set/unset git config variables

Instead of using construct such as:
    test_when_finished "git config --unset <key>"
    git config <key> <value>
uses
    test_config <key> <value>
The latter takes care of removing <key> at the end of the test.

Additionally, instead of
     git config <key> ""
or
     git config --unset <key>
uses
     test_unconfig <key>
The latter doesn't failed if <key> is not defined.

Signed-off-by: Yann Droneaud <redacted>
Signed-off-by: Junio C Hamano <redacted>