[minor] t1004-read-tree-m-u-wf.sh no more passes when "root"

Hello,

as I've been used to building and testing Git as root - granted, this 
goes against what INSTALL guidelines suggest - I have noticed the 
following : until v1.4.4.4, 'make test' could be successfully run as 
"root" ; it now (v1.5.5.1) fails on t1004-read-tree-m-u-wf.sh with 
following output:

* FAIL 13: funny symlink in work tree, un-unlink-able


                 rm -fr a b &&
                 git reset --hard &&

                 git checkout sym-a &&
                 chmod a-w a &&
                 test_must_fail git read-tree -m -u sym-a sym-a sym-b

Meanwhile I have found out that this isn't a problem for me in practice, 
since the test-suite passes when run "as myself", still I report this in 
case it could be a clue for something more serious...

Thanks,
Jérôme