Fix non-empty $CDPATH causing failed tests.
If CDPATH contains '.', every cd command prints the target path,
which causes additional output in tests and makes them fail.
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
removing b
created new head
% should abort
abort: crosses branches (use 'hg merge' or 'hg update -C' to discard changes)
% should succeed
abort: crosses branches (use 'hg merge' or 'hg update -C')