fugitive: Use FugitiveHead() instead of fugitive#head

function if possible and use it instead of the fugitive#head
(which will always autload the fugitive plugin).

autoload/airline/util.vim

@@ -105,7 +105,7 @@ function! airline#util#ignore_buf(name)
 endfunction
 
 function! airline#util#has_fugitive()
-  return exists('*fugitive#head') || exists('*FugitiveStatusline')
+  return exists('*fugitive#head') || exists('*FugitiveHead')
 endfunction
 
 function! airline#util#has_lawrencium()