diff: add option to skip resolving diff statuses

diff: add option to skip resolving diff statuses

By default, `diffcore_std()` resolves the statuses for queued diff file
pairs by calling `diff_resolve_rename_copy()`. If status information is
already manually set, invoking `diffcore_std()` may change the status
value.

Introduce the `skip_resolving_statuses` diff option that prevents
`diffcore_std()` from resolving file pair statuses when enabled.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>