JIT: Execute external command lazily based on the fallbacks order

[adaszko]

For a grammar cmd (--foo | {{{ echo bar }}});, the {{{ echo bar }}} command is executed even when we're matching the first word (i.e. cmd <TAB>), which is premature. {{{ echo bar }}} should be executed lazily only after we know --foo hasn't matched.

The output currently produced:

__complgen_jit () {
    local -a matches=()
    local -a completions=("bar" "foo")
    compadd -Q -a completions
    compadd -O matches -a completions
    [[ ${#matches} -gt 0 ]] && return
}
__complgen_jit

whereas it should look more like (in this case for ZSH)

__complgen_jit () {
    local -a matches=()
    local -a completions=("foo")
    compadd -Q -a completions
    compadd -O matches -a completions
    [[ ${#matches} -gt 0 ]] && return

    completions=("${(@f)$(echo bar)}")
    compadd -Q -a completions
    compadd -O matches -a completions
    [[ ${#matches} -gt 0 ]] && return
}
__complgen_jit

This piece of code is responsible for generating those completions: https://github.com/adaszko/complgen/blob/5e29dcbf46840bd9eb0599dcf737ce786d26340c/src/jit.rs#L395-L400