convert: fix leaking config strings

convert: fix leaking config strings

In `read_convert_config()`, we end up reading some string values into
variables. We don't free any potentially-existing old values though,
which will result in a memory leak in case the same key has been defined
multiple times.

Fix those leaks.

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