config: plug various memory leaks

Now that memory ownership rules around `git_config_string()` and
`git_config_pathname()` are clearer, it also got easier to spot that
the returned memory needs to be free'd. Plug a subset of those cases and
mark now-passing tests as leak free.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4060-diff-submodule-option-diff-format.sh

@@ -10,6 +10,7 @@ test_description='Support for diff format verbose submodule difference in git di
 This test tries to verify the sanity of --submodule=diff option of git diff.
 '
 
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 # Tested non-UTF-8 encoding