improve undotree integration; use exposed statusline functions.

autoload/airline/extensions.vim

@@ -14,7 +14,6 @@ let s:filetype_overrides = {
       \ 'netrw': [ 'netrw', '%f' ],
       \ 'unite': [ 'Unite', '%{unite#get_status_string()}' ],
       \ 'nerdtree': [ 'NERD', '' ],
-      \ 'undotree': [ 'undotree', '' ],
       \ 'gundo': [ 'Gundo', '' ],
       \ 'diff': [ 'diff', '' ],
       \ 'vimfiler': [ 'vimfiler', '%{vimfiler#get_status_string()}' ],
@@ -125,6 +124,10 @@ function! airline#extensions#load()
     call airline#extensions#commandt#init(s:ext)
   endif
 
+  if exists(':UndotreeToggle')
+    call airline#extensions#undotree#init(s:ext)
+  endif
+
   if g:airline_enable_tagbar && exists(':TagbarToggle')
     call airline#extensions#tagbar#init(s:ext)
   endif

autoload/airline/extensions/undotree.vim

@@ -0,0 +1,21 @@
+" MIT License. Copyright (c) 2013 Bailey Ling.
+" vim: et ts=2 sts=2 sw=2
+
+function! airline#extensions#undotree#apply()
+  if exists('t:undotree')
+    if &ft == 'undotree' && exists('*t:undotree.GetStatusLine')
+      call airline#extensions#apply_left_override('undo', '%{t:undotree.GetStatusLine()}')
+    else
+      call airline#extensions#apply_left_override('undotree', '%f')
+    endif
+
+    if &ft == 'diff' && exists('*t:diffpanel.GetStatusLine')
+      call airline#extensions#apply_left_override('diff', '%{t:diffpanel.GetStatusLine()}')
+    endif
+  endif
+endfunction
+
+function! airline#extensions#undotree#init(ext)
+  call a:ext.add_statusline_funcref(function('airline#extensions#undotree#apply'))
+endfunction
+