tindex.py: use a tmpdir in t/tmp instead of working directly in lib/bup/t/.
tindex.py: use a tmpdir in t/tmp instead of working directly in lib/bup/t/.

Signed-off-by: Rob Browning <1dc5917c2adeeba13d1ef33dbb40691a1b67cc65@defaultvalue.org>

gevaerts/bup,pombredanne/bup,gevaerts/bup,jbaber/bup,mhoeher/bup,jbaber/bup,tjanez/bup,jbaber/bup,jbaber/bup,jbaber/bup,pombredanne/bup,tjanez/bup,jbaber/bup,gevaerts/bup,pombredanne/bup,gevaerts/bup,ToxicFrog/bup,tjanez/bup,ToxicFrog/bup,mhoeher/bup,mhoeher/bup,mhoeher/bup,pombredanne/bup,mhoeher/bup,ToxicFrog/bup,ToxicFrog/bup,tjanez/bup,pombredanne/bup,mhoeher/bup,tjanez/bup,gevaerts/bup,pombredanne/bup,ToxicFrog/bup,tjanez/bup