Deal more gracefully with unusual display changesets

Summary: These came up in dealing with the diff produced by T271. When a file is
unmodified, don't try to use the "ignore all whitespace" algorithm on it. Also,
detect "changed only by adding or removing trailing whitespace" vs "this file
was not modified" correctly.
Test Plan: Viewed the diff that came out of running 'arc diff' on my T271 mess.
Reviewed By: tuomaspelkonen
Reviewers: alex, jungejason, tuomaspelkonen, aran
CC: aran, tuomaspelkonen
Differential Revision: 548