Don't ignore internal whitespace changes in "Whitespace: Ignore All"

Summary:
When whitespace changes between two non-whitespace characters (e.g., in a
string), always treat it as a change.

Test Plan:
Disabled render cache, made internal and external whitespace changes, rendered a
diff, got internal change always marked and external change marked correctly
depending on mode.

Reviewed By: tuomaspelkonen
Reviewers: tuomaspelkonen
CC: aran, tuomaspelkonen, epriestley
Differential Revision: 403