builtin/pull: fix leaking "ff" option

builtin/pull: fix leaking "ff" option

The `opt_ff` field gets populated either via `OPT_PASSTHRU` via
`config_get_ff()` or when `--rebase` is passed. So we sometimes end up
overriding the value in `opt_ff` with another value, but we do not free
the old value, causing a memory leak.

Adapt the type of the variable to be `char *` and consistently assign
allocated strings to it such that we can easily free it when it is being
overridden.

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