[minor] two tests broken when run with a --root directory that's a symlink

These issues are minor, I noticed it because I test with /dev/shm/git
as the --root, which on Debian is symlinked to /run/..

    $ rm -rf /tmp/{foo,bar}
    $ mkdir /tmp/target; ln -s /tmp/target /tmp/link
    $ prove ./t4035-diff-quiet.sh ./t9903-bash-prompt.sh :: --root=/tmp/target
    ./t4035-diff-quiet.sh ... ok
    ./t9903-bash-prompt.sh .. ok
    All tests successful.
    Files=2, Tests=64,  1 wallclock secs ( 0.04 usr  0.00 sys +  0.07
cusr  0.06 csys =  0.17 CPU)
    Result: PASS
    $ prove ./t4035-diff-quiet.sh ./t9903-bash-prompt.sh :: --root=/tmp/link
    ./t4035-diff-quiet.sh ... Dubious, test returned 1 (wstat 256, 0x100)
    Failed 3/20 subtests
    ./t9903-bash-prompt.sh .. Dubious, test returned 1 (wstat 256, 0x100)
    Failed 6/44 subtests

Everything else in the test suite passes with a --root that's a symlink.