use different face for visited `occur' line numbers

[Drew Adams]

Possible minor enhancement, for after the release:

In Info, two different faces are used for links: `info-xref' and
`info-xref-visited'. In `occur', it would be useful to do the same thing:
Indicate whether a given line has been visited previously by fontifying the
line number differently.

I do this in my own code, and I find it helps quite a bit. Otherwise, it's
easy to click a line by mistake that you've already visited, especially when
lines have the same or similar text. Attached is a screenshot, to show what
I mean.

occur-line-numbers.jpg
Description: JPEG image