builtin/fast-export: fix leaking diff options

builtin/fast-export: fix leaking diff options

Before calling `handle_commit()` in a loop, we set `diffopt.no_free`
such that its contents aren't getting freed inside of `handle_commit()`.
We never unset that flag though, which means that the structure's
allocated resources will ultimately leak.

Fix this by unsetting the flag after the loop such that we release its
resources via `release_revisions()`.

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