Changes the implementation of the mergeInto method of the class
NavigationHistoryEntry. This is to fix the reported github issue #339,
where a user has multiple different editors whose EditorInputs are both
FileEditorInputs with the same underlying file. Thus the editorInputs of
the multiple editors were considered the same, and when navigating
between the editors, the navigation history entries were being merged,
incorrectly.

The supplied change checks both if the editor inputs and editorIDs of
two navigation history entries are the same before merging the two
navigation history entries.