Publishing: Fix bug with finding project for a file.

Publishing:  Fix bug with finding project for a file.

Ian Barton writes:

>
> This has been happening for a while. I suspect it's something in my
> settings, but I can't work out what. When I publish a single file, I
> get the following error:
>
> Debugger entered--Lisp error: (wrong-type-argument arrayp nil)
>  file-truename(nil)
> org-publish-file("/home/ian/nfs/firewall/Documents/org/holiday/holiday.org")
>  org-publish-current-file(nil)
>  call-interactively(org-publish-current-file)
>  org-export(nil)
>  call-interactively(org-export)
>
> Publishing a whole project works fine.

This was caused by an attempt of the publishing code to always find
the top enclosing project.  While this makes sense when publishing a
project, for a single file we want just the smallest enclosing
project.  This commit makes sure that this difference is treated
correctly.