Customize tab titles in atom text editor.

init.coffee

# place this snippet into init.coffee in ~/.atom directory

atom.workspace.observeTextEditors (editor) ->
    if editor.getTitle() isnt "untitled"
        sp = editor.getPath().split('/')
        title = sp.slice(sp.length-2).join('/')
        editor.getTitle = -> title
        editor.getLongTitle = -> title

for item in atom.workspace.getPaneItems()
    if item.emitter?
        item.emitter.emit "did-change-title", item.getTitle()

Please note if anyone needs it just because of multiple 'index.…' tabs, a CSS solution is possible too
[in Atom -> Stylesheet add:]

.tab-bar .tab .title[data-name^="index"] {
letter-spacing: -1px;
direction: rtl;
text-align: left;
}