--exclude-if-present and --exclude-other-filesystems causes crash with inaccessible other fs

[az]

this is a forward of debian bug #685352, which lives over there: http://bugs.debian.org/685352

the original submitter has a fuse filesystem mounted (=inaccessible to anybody else, incl. root)
and a symlink pointing to it, and wants to back stuff up as root - except the inaccessible stuff of course,
so he uses --exclude-other-filesystems.

this works fine if only --exclude-other-filesystems is specified, but if --exclude-if-present is also active,
then the backup crashes because duplicity checks for the presence of the testfile in the symlinked
dir (=inaccessible), and doesn't properly deal with the error code (line 504 in path.py covers only enodir, enoent, but not eaccess).

to reproduce:
mkdir testdir && cd testdir
touch bla
ln -s /root # or any other dir that has no x perms for you, and is on a different fs

duplicity --exclude-if-present .whatever --exclude-other-filesystems file:///tmp/testme

regards
az

[az (az-debian)]

i've tracked down the root cause and have a fix for it:

if --exclude-if-present is used, then the path to check is run through path.append which creates a new
path object which in turn uses setdata which in turn follows links. the rest of path.py is careful not
to follow any symlinks, but the constructor isn't.