Substitution commands can replace text with empty string

Closes #488

src/packages/command-panel/lib/commands/composite-command.coffee

@@ -40,7 +40,7 @@ class CompositeCommand
               if bufferRanges.length and not currentCommand.preserveSelections
                 editSession.setSelectedBufferRanges(bufferRanges, autoscroll: true)
 
-          operationsWillChangeBuffer = _.detect(operations, (operation) -> operation.newText)
+          operationsWillChangeBuffer = _.detect(operations, (operation) -> operation.newText?)
 
           if operationsWillChangeBuffer
             editSession.transact(executeOperations)

src/packages/command-panel/lib/operation.coffee

@@ -19,7 +19,7 @@ class Operation
     @getMarker().getRange()
 
   execute: (editSession) ->
-    @getBuffer().change(@getBufferRange(), @newText) if @newText
+    @getBuffer().change(@getBufferRange(), @newText) if @newText?
     @getBufferRange() unless @preserveSelection
 
   preview: ->

src/packages/command-panel/spec/command-interpreter-spec.coffee

@@ -426,6 +426,12 @@ describe "CommandInterpreter", ->
       runs ->
         expect(buffer.lineForRow(6)).toBe '\t\t\tcurrent < pivot ? left.push(current) : right.push(current);'
 
+    it "removes matched text when an empty string is given as the replacement", ->
+      waitsForPromise ->
+        interpreter.eval('s/items//', editSession)
+      runs ->
+        expect(buffer.lineForRow(1)).toBe('  var sort = function() {')
+
   describe "X x/regex/", ->
     it "returns selection operations for all regex matches in all the project's files", ->
       editSession.destroy()