Simplify hunk parsing

Summary:
Simplify whitespace-ignoring diffing:

• Currently, we generate two diffs (one ignoring whitespace, one normal) and then copy the text content from the normal one to the whitespace-ignoring one.
• Instead, copy the change types from the ignoring one to the normal one.
  • This is cheaper and much simpler.
  • It means that we have the right change types in reparseHunksForSpecialAttributes(), and can simplify some other things.

Simplify whitespace changes, unchanged files, and deleted file detections:

• Currently, we do this inline with a bunch of other stuff, in the reparse step.
• Instead, do it separately. This is simpler.

Simplify intraline whitespace handling:

• Currently, this is a complicated mess that makes roughly zero sense.
• Instead, do it in reparse in a straightforward way.

Partially fix handling of files changed only by changing whitespace.
Partially fix handling of unchanged files.

Test Plan:

• Ran unit tests.
• Created context-less diffs, verified they rendered reasonably.
• Generated a diff with prefix whitespace, suffix whitespace, intraline whitespace, and non-whitespace changes. Verified changes rendered correctly in "ignore most" and "show all" modes.
• Verified unchanged files and files with only whitspace changes render with the correct masks.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D4398