Don't prompt to save on close if buffer is opened elsewhere

Only prompt to save when the buffer is dirty and the last
open session on it is being destroyed.

spec/app/editor-spec.coffee

@@ -199,6 +199,15 @@ describe "Editor", ->
         expect(editor.remove).not.toHaveBeenCalled()
         expect(atom.confirm).toHaveBeenCalled()
 
+      it "doesn't trigger an alert if the buffer is opened in multiple sessions", ->
+        spyOn(editor, 'remove').andCallThrough()
+        spyOn(atom, 'confirm')
+        editor.insertText("I AM CHANGED!")
+        editor.splitLeft()
+        editor.trigger "core:close"
+        expect(editor.remove).toHaveBeenCalled()
+        expect(atom.confirm).not.toHaveBeenCalled()
+
   describe ".edit(editSession)", ->
     otherEditSession = null
 

src/app/buffer.coffee

@@ -54,6 +54,8 @@ class Buffer
     @destroy() if @refcount <= 0
     this
 
+  hasEditors: -> @refcount > 1
+
   subscribeToFile: ->
     @file.on "contents-changed", =>
       if @isModified()

src/app/edit-session.coffee

@@ -142,6 +142,8 @@ class EditSession
   lineLengthForBufferRow: (row) -> @buffer.lineLengthForRow(row)
   scanInRange: (args...) -> @buffer.scanInRange(args...)
   backwardsScanInRange: (args...) -> @buffer.backwardsScanInRange(args...)
+  isModified: -> @buffer.isModified()
+  hasEditors: -> @buffer.hasEditors()
 
   screenPositionForBufferPosition: (bufferPosition, options) -> @displayBuffer.screenPositionForBufferPosition(bufferPosition, options)
   bufferPositionForScreenPosition: (screenPosition, options) -> @displayBuffer.bufferPositionForScreenPosition(screenPosition, options)

src/app/editor.coffee

@@ -463,7 +463,7 @@ class Editor extends View
       @remove() if @editSessions.length is 0
       callback(index) if callback
 
-    if editSession.buffer.isModified()
+    if editSession.isModified() and not editSession.hasEditors()
       @promptToSaveDirtySession(editSession, destroySession)
     else
       destroySession(editSession)