Reverting "fix `ido-merge-work-directories`"
Revert "fix `ido-merge-work-directories`"

This reverts commit a1ac8f97ecb4a0cbb4764210048484fe96074c65.
2nd try fix for `ido-merge-work-directories`

- items are cons cells of (basename . dirname)
- match should be performed against basename
- caching is skipped for the merge case
- flx highlighting is disabled for merged case

#10