path->pagenode returns incorrect path if starting-path is an immediate relative filename

[Stefan Schmid]

> (path->pagenode "assets" 'index.html)
'../assets

(dirname path) (respectively (split-path path)) returns 'relative for an immediate relative filename.
(path->pagenode p [starting-path]) should return p if (eq? starting-dir 'relative).

[Matthew Butterick]

Thanks!