/usr/share/bash-completion/completions/debsign is in devscripts 2.17.6+deb9u2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 | # /usr/share/bash-completion/completions/debsign
# Bash command completion for ‘debsign(1)’.
# Documentation: ‘bash(1)’, section “Programmable Completion”.
_debsign()
{
    local cur prev options
    COMPREPLY=()
    cur=${COMP_WORDS[COMP_CWORD]}
    prev=${COMP_WORDS[COMP_CWORD-1]}
    options='-r -p -m -e -k -S -a -t --multi -h --help -version \
             --re-sign --no-re-sign --debs-dir --no-conf --noconf'
    case $prev in
        -k)
            keyid_options=`gpg -K|grep ^sec|cut -d'/' -f2|cut -d' ' -f1`
            COMPREPLY=( $( compgen -W "$keyid_options" | grep ^$cur ) )
            ;;
        *)
            COMPREPLY=( $(
                    compgen -f -X "!*.changes"   -- "$cur"
                    compgen -f -X "!*.buildinfo" -- "$cur"
                    compgen -f -X "!*.dsc"       -- "$cur"
                    compgen -f -X "!*.commands"  -- "$cur"
                    compgen -W "$options" | grep "^$cur"
                ) )
            ;;
    esac
    return 0
}
complete -F _debsign -o filenames debsign
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