Add 'exclude' action for excluding current/selected items from the result

[junegunn]

Close #4185

Summary

Add exclude action to make the current item or the selected items excluded from the result.

Discussion / Issues

• exclude "excludes" the selected items (--multi) at once if there's any, instead of excluding the "current" item on the cursor. Is this acceptable, or should we have a separate action? i.e. exclude-current + exclude-selected. I prefer the current implementation.
  • Decided to add exclude and exclude-multi
• The total item count does not decrease on exclude. I think this has a value of indicating that some items are excluded, as opposed to the list being fully "reloaded."
• To avoid the overhead of locking, we copy the exclusion list before passing it to the matcher. This is fine for small lists. But it does have performance problem if the exclusion list becomes excessively large.

  # But realistically, who would do this?
  seq 10000000 | fzf --sync --multi --bind start:select-all+exclude-multi

[junegunn]

I'll be renaming it to drop or exclude.

[junegunn]

On second thought, I think we need exclude-current that doesn't affect the selection.

• You are given a list, and you want to select some items.
• You carefully go through the list, select items one by one.
• You sometimes want to remove the items that you're never going to choose from the list.
  • But you don't want to lose the previous selections!

[junegunn]

On third thought, I think we should have exclude and exclude-multi instead of exclude-current and exclude.