builtin/revert: fix leaking `gpg_sign` and `strategy` config

builtin/revert: fix leaking `gpg_sign` and `strategy` config

We leak the config values when `gpg_sign` or `strategy` options are
being overridden via the command line. To fix this we need to free the
old value, which requires us to figure out whether the value was changed
via an option in the first place. The easy way to do this, which is to
initialize local variables with `NULL`, doesn't work because we cannot
tell the case where the user has passed e.g. `--no-gpg-sign`. Instead,
we use a sentinel value for both values that we can compare against to
check whether the user has passed the option.

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