Integrate external extended bash-completion script

The [bash-completion] git repository has had an extended version for
dealing with bash-compleions of our `secret-tool` for a while now.

We've agreed it makes sense to ship their version upstream, i.e. in this
repo. I didn't author this, the real credits go to the authors mentioned
below.

[bash-completion]: https://github.com/scop/bash-completion

Co-authored-by: Ville Skyttä <ville.skytta@iki.fi>
Co-authored-by: Koichi Murase <myoga.murase@gmail.com>
Fixes: https://gitlab.gnome.org/GNOME/libsecret/-/issues/103

bash-completion/secret-tool

@@ -1,19 +1,50 @@
-# secret-tool(1) completion
+# bash completion for secret-tool(1)                       -*- shell-script -*-
 
-_secret-tool()
+_comp_cmd_secret_tool()
 {
-    local cur prev word cword
+    local cur prev words cword was_split comp_args
-    _init_completion || return
+    _comp_initialize -s -- "$@" || return
 
-    if [[ $cur == -* ]]; then
+    [[ $was_split ]] && return
-      local opts="--help --verbose -q --quiet"
+
-      COMPREPLY=( $(compgen -W "$opts" -- "$cur") )
+    local -i i
-      return
+    local mode="" has_mode="" word
-    elif [[ $cword -eq 1 ]]; then
+    for i in ${!words[*]}; do
-      local commands='store lookup clear search'
+        if [[ $i -gt 0 && ${words[i]} != -* ]]; then
-      COMPREPLY=( $(compgen -W "$commands" -- "$cur") )
+            ((i != cword)) && mode=${words[i]} has_mode=set
+            break
+        fi
+    done
+    if [[ ! $has_mode ]]; then
+        # generate modes
+        _comp_compgen_split -- "$("$1" nonexistent-mode 2>&1 |
+            while read -r first second third rest; do
+                if [[ $first == "${1##*/}" ]]; then
+                    printf "%s\n" "$second"
+                elif [[ $first == usage: && $second == "${1##*/}" ]]; then
+                    printf "%s\n" "$third"
+                fi
+            done)"
+        return
     fi
+
+    case $mode in
+        store)
+            if [[ ${words[*]} != *\ --label[\ =]* ]]; then
+                _comp_compgen -- -W "--label="
+                [[ ${COMPREPLY-} == *= ]] && compopt -o nospace
+            fi
+            ;;
+        search)
+            local -A opts=([--all]="" [--unlock]="")
+            for word in "${words[@]:2}"; do
+                [[ $word ]] && unset -v 'opts[$word]'
+            done
+            ((${#opts[@]})) &&
+                _comp_compgen -- -W '"${!opts[@]}"'
+            ;;
+    esac
 } &&
-complete -F _secret-tool secret-tool
+    complete -F _comp_cmd_secret_tool secret-tool
 
 # ex: filetype=sh