Formatter: Use pathshorten() instead of substitute()

The default formatter uses a hand-build regexp for shortening the path
in the tabline. However, since it uses the \w  regex atom, this won't
match e.g. cyrillic letters.

To fix this, use the builtin pathshorten() function which does handle
this case correctly. For a test, use e.g. 'D/Учёба/t.c'

closes #1737

autoload/airline/extensions/tabline/formatters/default.vim

@@ -18,7 +18,9 @@ function! airline#extensions#tabline#formatters#default#format(bufnr, buffers)
     let _ .= '[No Name]'
   else
     if s:fnamecollapse
-      let _ .= substitute(fnamemodify(name, fmod), '\v\w\zs.{-}\ze(\\|/)', '', 'g')
+      " Does not handle non-ascii characters like Cyrillic: 'D/Учёба/t.c'
+      "let _ .= substitute(fnamemodify(name, fmod), '\v\w\zs.{-}\ze(\\|/)', '', 'g')
+      let _ .= pathshorten(fnamemodify(name, fmod))
     else
       let _ .= fnamemodify(name, fmod)
     endif