[bash-completion] Move -F/_fzf filter to __fzf_orig_completion_filter
[bash-completion] Move -F/_fzf filter to __fzf_orig_completion_filter

This prevents mistakes like the one fixed by the previous commit, and
also speeds bash startup a tiny bit:

before:

    $ HISTFILE=/tmp/bashhist hyperfine 'bash --rcfile shell/completion.bash -i'
    Benchmark #1: bash --rcfile shell/completion.bash -i
      Time (mean ± σ):      22.4 ms ±   0.6 ms    [User: 28.7 ms, System: 7.8 ms]
      Range (min … max):    21.7 ms …  25.2 ms    123 runs

after:

    $ HISTFILE=/tmp/bashhist hyperfine 'bash --rcfile shell/completion.bash -i'
    Benchmark #1: bash --rcfile shell/completion.bash -i
      Time (mean ± σ):      21.2 ms ±   0.3 ms    [User: 24.9 ms, System: 6.4 ms]
      Range (min … max):    20.7 ms …  23.3 ms    132 runs

janlazo/fzf,junegunn/fzf,junegunn/fzf,junegunn/fzf,janlazo/fzf,janlazo/fzf