If a file in the srcdir is removed, exposing a file in the underlaydir,
ikiwiki will not notice the removal, and the
page from the underlay will not be built. (However, it will be if the wiki
gets rebuilt.)

> This problem is caused by ikiwiki storing only filenames relative to
> the srcdir or underlay, and mtime comparison not handling this case.

> A related problem occurs if changing a site's theme with the
> [[plugins/theme]] plugin. The style.css of the old and new theme
> often has the same mtime, so ikiwiki does not update it w/o a rebuild.
> This is worked around in theme.pm with a special-purpose needsbuild hook.
> --[[Joey]]
