Fix absolute paths with prepare-dev.py
Fix absolute paths with prepare-dev.py

Stephen Gallagher's recent change to install sites into a default directory had
the unintentional side effect of breaking prepare-dev.py. The issue was that
using the Site object with a relative path now tries to use that path relative
to the default directory rather than the current working directory.

This change makes the relative 'reviewboard' path absolute before constructing
the Site object.

Testing done:
Ran prepare-dev.py. Before this change it would error out with permissions.
After, it runs successfully.

Reviewed at https://reviews.reviewboard.org/r/6756/

sgallagher/reviewboard,custode/reviewboard,chipx86/reviewboard,KnowNo/reviewboard,custode/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,custode/reviewboard,sgallagher/reviewboard,bkochendorfer/reviewboard,davidt/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,davidt/reviewboard,bkochendorfer/reviewboard,sgallagher/reviewboard,reviewboard/reviewboard,KnowNo/reviewboard,davidt/reviewboard,KnowNo/reviewboard,brennie/reviewboard,brennie/reviewboard,KnowNo/reviewboard,bkochendorfer/reviewboard,beol/reviewboard,chipx86/reviewboard,brennie/reviewboard,bkochendorfer/reviewboard,brennie/reviewboard,davidt/reviewboard,beol/reviewboard,beol/reviewboard,custode/reviewboard,sgallagher/reviewboard,beol/reviewboard