jekyll doctor should print a warning if two permalinks only differ by case

[akoeplinger]

(This is a continuation from #3034)

If I'm on a case-insensitive file system and create two pages with permalinks that only differ by case, the site generation overwrites one of the pages. This should result in a jekyll doctor warning.

[parkr]

Case insensitive file system? What kind of tom-foolery is this?!

I don't actually know how Ruby would know whether you're on a case-insensitive filesystem... so not sure how we would show this. But you can give it a shot if you want!

[akoeplinger]

I don't think it'd actually need to know if I'm on a case-insensitive FS right now, as someone else might be and run into a problem (as I showed in jekyll/jekyll-redirect-from#51). So I'd say actively discourage two permalinks whose strings are equal-ignore-case.

[parkr]

If you open up _site on that system, do you see download and Download directories? If so, it's a WEBrick problem. URL's are case-insensitive, I think.

[akoeplinger]

On a case-insensitive FS there's no way to have a download and a Download folder, you'd get an exception when trying to create the second one. WEBrick just serves what it sees on the FS, so if you're on a case-insensitive FS the URLs are case-insensitive too, otherwise they are not, like on Linux.

[parkr]

Want to code this up?

[akoeplinger]

@parkr Here you go: #3171