Differential defaults to 'ignore-trailing' until indentation issues are solved.

Summary:
'ignore-all' is very confusing, especially for Python, because the indentation
is incorrect.

Test Plan:
Tested loading D240353 without any options and made sure 'ignore-trailing' was
selected and the diff looked correct.

Reviewed By: aran
Reviewers: jungejason, simpkins, aran, dpepper
Commenters: dpepper
CC: grglr, aran, dpepper, tuomaspelkonen
Differential Revision: 171