bug#12230: 24.2; revert-buffer in an Info buffer should refresh Info-index-nodes

[Juri Linkov]

> The problem is that info.el caches the Index nodes in the alist stored
> in Info-index-nodes.  It should therefore remove from that alist the
> association of the Info file whose buffer is reverted.

Since `Info-revert-find-node' is not the right place to clear
the cache because the same problem can occur after killing
the Info buffer and revisiting the same Info manual manually,
perhaps the right fix would be to check the modtime of the Info file
in `Info-find-file' and clear the cache for modified files.