t4304: use test_config to set/unset git config variables
t4304: use test_config 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.

Tests are modified to assume correct (default) configuration at entry,
and to reset the modified configuration variables at the end.

Signed-off-by: Yann Droneaud <32f7bf5a1f07a8b363256f1773dbeada59d65244@opteya.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git