If you enabled core.safecrlf on an LF-ending platform, we would
error even for files with all LFs. We should only be warning on
irreversible mappings, I think.
Diff and status do not want core.safecrlf to actually raise an
error regardless of the setting, so this extends the filter API
with an additional options flags parameter and adds a flag so that
filters can be applied with GIT_FILTER_OPT_ALLOW_UNSAFE, indicating
that unsafe filter application should be downgraded from a failure
to a warning.
