t9903: Don't fail when run from path accessed through symlink

t9903: Don't fail when run from path accessed through symlink

When the git directory is accessed through a symlink like

  ln -s /tmp/git /tmp/git-symlink
  cd /tmp/git-symlink/t
  make -C .. && ./t9903-bash-prompt.sh

$TRASH_DIRECTORY is /tmp/git-symlink/t/trash directory.t9903-bash-prompt
and $(pwd -P) is /tmp/git/t/trash directory.t9903-bash-prompt.

When __gitdir looks up the path through 'git rev-parse --git-dir', it
will return paths similar to $(pwd -P). This behavior is already tested in
t9903 'gitdir - resulting path avoids symlinks'.

Signed-off-by: Torstein Hegge <hegge@resisty.net>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>