Render line decorations.
Use _.deepContains
Upgrade to git-diff 0.34.0 to fix spec
:lipstick:
Merge pull request #2682 from atom/bo-line-decorations

Render line decorations
Index the previous decoration cache by lineNumberId rather than screenRow

Why? Screen rows change. If some operation (folding?) changes the 
screen rows and the decorations at the same time, the previous 
decorations will no longer be valid and can no longer be diffed against 
the decorations to-be-rendered.
Remove cached decorations for removed lines
Set the `decorations` and `previousDecorations` vars before loops

Both loops use both vars, so both need to be available before the 
diffing!
Use ? in the conditional.
Merge pull request #2693 from atom/bo-fix-gutter-prev-decorations

Fix caching of gutter's previous decorations
Add command to select text using page up/down keys
Add text selection using page up/down keys to the React-based editor
Implement select page up/down in Editor
Add spec for Editor::selectPageUp/Down
:memo: Correct spec description