Don't use the "ignore all whitespace" algorithm on multi-hunk diffs

Summary:
Diffs with missing context don't render properly in the "ignore all whitespace"
algorith, so don't try to use it. These diffs can occur if someone creates a
diff via the web interface, for example, or if they muck around in their copy of
'arc'.

See D473, T246 (a problem with D473), rPe5bb756b5191720 (revert of D473) and
T231.

Test Plan:
Viewed a diff with missing context from the web interface. Verified normal diffs
still rendered with all whitespace ignored.

Reviewed By: fratrik
Reviewers: jungejason, aran, tuomaspelkonen, fratrik
Commenters: jungejason
CC: aran, epriestley, fratrik, jungejason
Differential Revision: 500