diff-lib: fix leaking diffopts in `do_diff_cache()`

diff-lib: fix leaking diffopts in `do_diff_cache()`

In `do_diff_cache()` we initialize a new `rev_info` and then overwrite
its `diffopt` with a user-provided set of options. This can leak memory
because `repo_init_revisions()` may end up allocating memory for the
`diffopt` itself depending on the configuration. And since that field is
overwritten we won't ever free it.

Plug the memory leak by releasing the diffopts before we overwrite them.

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