Isolate folds on a per-display-buffer basis

Creating a fold in one pane now has no effect on an edit session
for the same buffer in another pane. Before they were polluting each
other by sharing the same fold markers.

spec/app/display-buffer-spec.coffee

@@ -257,6 +257,12 @@ describe "DisplayBuffer", ->
           expect(line0.fold).toBe outerFold
           expect(line1.fold).toBeUndefined()
 
+      describe "when there is another display buffer pointing to the same buffer", ->
+        it "does not create folds in the other display buffer", ->
+          otherDisplayBuffer = new DisplayBuffer(buffer, { tabLength })
+          displayBuffer.createFold(2, 4)
+          expect(otherDisplayBuffer.foldsStartingAtBufferRow(2).length).toBe 0
+
     describe "when the buffer changes", ->
       [fold1, fold2] = []
       beforeEach ->