correctly handle the stow/target directories as non-canonical paths

Fix the case discovered by Hiroyuki Iwatsuki where stowing fails if
the stow / target directories are non-canonical paths.  For example,
on FreeBSD /home is a symlink pointing to 'usr/home', so running with
the stow directory as /home/user/local/stow and the target directory
as /home/user/local previously resulted in the stow directory path
being calculated as ../../../usr/home/user/local/stow relative to
the target.

http://article.gmane.org/gmane.comp.gnu.stow.bugs/8820
add TODO for install-hooks