Move code related to the diff lines into DiffBuilder.
Refactor some of the diff creation logic into methods.
Refactor parts of DiffBuilder#emitLine into methods.
Move syntax language selection into emitLine.
Move more logic out of DiffBuilder#createDiff.
Clear highlightData when adding context lines.
Move move/copy header line logic out of createDiff.
Move diff caching into PatchFile instead of in the view.
Move embedded language selection out of emitLine.
Rename messageForLine -> messagesForLine.
Micro optimize <cr-action>.

This saves 6ms when expanding the EventHandler.cpp diff
on https://codereview.chromium.org/356673002

First step in issue #63.