extract autoshow logic to its own file

autoload/airline/extensions/tabline/autoshow.vim

@@ -0,0 +1,50 @@
+" MIT License. Copyright (c) 2013-2015 Bailey Ling.
+" vim: et ts=2 sts=2 sw=2
+
+let s:buf_min_count = get(g:, 'airline#extensions#tabline#buffer_min_count', 0)
+let s:tab_min_count = get(g:, 'airline#extensions#tabline#tab_min_count', 0)
+
+function! airline#extensions#tabline#autoshow#off()
+  if exists('s:original_tabline')
+    let &tabline = s:original_tabline
+    let &showtabline = s:original_showtabline
+  endif
+
+  augroup airline_tabline_autoshow
+    autocmd!
+  augroup END
+endfunction
+
+function! airline#extensions#tabline#autoshow#on()
+  let [ s:original_tabline, s:original_showtabline ] = [ &tabline, &showtabline ]
+
+  augroup airline_tabline_autoshow
+    autocmd!
+    if s:buf_min_count <= 0 && s:tab_min_count <= 1
+      set showtabline=2
+    else
+      if s:show_buffers == 1
+        autocmd BufEnter  * call <sid>show_tabline(s:buf_min_count, len(airline#extensions#tabline#buflist#list()))
+        autocmd BufUnload * call <sid>show_tabline(s:buf_min_count, len(airline#extensions#tabline#buflist#list()) - 1)
+      else
+        autocmd TabEnter  * call <sid>show_tabline(s:tab_min_count, tabpagenr('$'))
+      endif
+    endif
+
+    " Invalidate cache.  This has to come after the BufUnload for
+    " s:show_buffers, to invalidate the cache for BufEnter.
+    autocmd BufAdd,BufUnload * call airline#extensions#tabline#buflist#invalidate()
+  augroup END
+endfunction
+
+function! s:show_tabline(min_count, total_count)
+  if a:total_count >= a:min_count
+    if &showtabline != 2
+      set showtabline=2
+    endif
+  else
+    if &showtabline != 0
+      set showtabline=0
+    endif
+  endif
+endfunction