tests: fix failures when CDPATH is set

Some tests would otherwise fail because commands such as
  cd trash/repos/foo && git rev-list --reverse HEAD | head -1
would return 2 lines instead of 1: the 'cd' command also
prints the path when CDPATH is set.

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>