plugin/magit.vim: clear undo history after each update_buffer() call (refs #64)

Then, user can't undo some changes in magit buffer made the backend (these changes would only be virtual).
But it can still undo some changes when he is typing.

Catch undo and make undo really undoing magit commands would be soooo cool, but seems quite difficult to implement.

plugin/magit.vim

@@ -547,6 +547,8 @@ function! magit#update_buffer()
 		call call(func.fn, func.arg)
 	endfor
 
+	call magit#utils#clear_undo()
+
 	call winrestview(l:winview)
 
 	if ( b:magit_current_commit_mode != '' )