Preserve the face foreground in Info-fontify-node

Preserve the face foreground in Info-fontify-node

* lisp/info.el (Info-fontify-node): Preserve the face foreground.
Previously `Info-index' added the `info-index-match' face to the
strings of the found index entries.  Later
`Info-virtual-index-find-node' inserts strings to the Info buffer.
And finally `Info-fontify-node' puts the `font-lock-face' property
with `info-xref' on links.  The `face info-index-match' takes
precedence over `font-lock-face info-xref' (bug#14645).