Disambiguate files with same name in the tab bar

[namandixit]

When multiple files are opened, the title of each tab is set equal to the name of the file. However, if two files have the same name, this makes thing confusing. One disambiguating scheme that could be used is to copy Emacs that scans each path back one folder at a time until it finds the folder whose name doesn't clash, and then adds that folder's name to the tab name. For example, two files with the path C:/a/b/c/d.txt and P:/a/e/c/d.txt will be shown as d.txt <b> and d.txt <e> respectively.

[ryanfleury]

Done in 9c1f8ef.