Don't destroy parts on cleanup

Destroying parts on cleanup can have weird effects, because we observe parts in
log view.

assets/scripts/app/models/log.coffee

@@ -33,8 +33,7 @@ require 'travis/log_chunks'
 
   clearParts: ->
     parts = @get('parts')
-    @notifyPropertyChange('parts')
-    parts.destroy()
+    parts.set('content', [])
 
   fetch: ->
     console.log 'log model: fetching log' if Log.DEBUG

assets/scripts/app/views/log.coffee

@@ -68,6 +68,7 @@ Travis.reopen
         parts = log.get('parts')
         parts.removeArrayObserver(@, didChange: 'partsDidChange', willChange: 'noop')
         parts.destroy()
+        log.notifyPropertyChange('parts')
         @lineSelector?.willDestroy()
 
     createEngine: ->