cmd: make auto completion work for all shells and reduce the size

This updates the bash completion to work with GenBashCompletionV2
which cuts down the size of the completion file dramatically.

See: https://forum.rclone.org/t/request-make-remote-path-completion-work-for-fish-and-zsh/42982/
See: #7000

cmd/help.go

@@ -38,58 +38,10 @@ documentation, changelog and configuration walkthroughs.
 		fs.Debugf("rclone", "Version %q finishing with parameters %q", fs.Version, os.Args)
 		atexit.Run()
 	},
-	BashCompletionFunction: bashCompletionFunc,
-	DisableAutoGenTag:      true,
+	ValidArgsFunction: validArgs,
+	DisableAutoGenTag: true,
 }
 
-const (
-	bashCompletionFunc = `
-__rclone_custom_func() {
-    if [[ ${#COMPREPLY[@]} -eq 0 ]]; then
-        local cur cword prev words
-        if declare -F _init_completion > /dev/null; then
-            _init_completion -n : || return
-        else
-            __rclone_init_completion -n : || return
-        fi
-	local rclone=(command rclone --ask-password=false)
-        if [[ $cur != *:* ]]; then
-            local ifs=$IFS
-            IFS=$'\n'
-            local remotes=($("${rclone[@]}" listremotes 2> /dev/null))
-            IFS=$ifs
-            local remote
-            for remote in "${remotes[@]}"; do
-                [[ $remote != $cur* ]] || COMPREPLY+=("$remote")
-            done
-            if [[ ${COMPREPLY[@]} ]]; then
-                local paths=("$cur"*)
-                [[ ! -f ${paths[0]} ]] || COMPREPLY+=("${paths[@]}")
-            fi
-        else
-            local path=${cur#*:}
-            if [[ $path == */* ]]; then
-                local prefix=$(eval printf '%s' "${path%/*}")
-            else
-                local prefix=
-            fi
-            local ifs=$IFS
-            IFS=$'\n'
-            local lines=($("${rclone[@]}" lsf "${cur%%:*}:$prefix" 2> /dev/null))
-            IFS=$ifs
-            local line
-            for line in "${lines[@]}"; do
-                local reply=${prefix:+$prefix/}$line
-                [[ $reply != $path* ]] || COMPREPLY+=("$reply")
-            done
-	    [[ ! ${COMPREPLY[@]} || $(type -t compopt) != builtin ]] || compopt -o filenames
-        fi
-        [[ ! ${COMPREPLY[@]} || $(type -t compopt) != builtin ]] || compopt -o nospace
-    fi
-}
-`
-)
-
 // GeneratingDocs is set by rclone gendocs to alter the format of the
 // output suitable for the documentation.
 var GeneratingDocs = false
@@ -220,10 +172,25 @@ func setupRootCommand(rootCmd *cobra.Command) {
 	helpCommand.AddCommand(helpBackends)
 	helpCommand.AddCommand(helpBackend)
 
+	// Set command completion for all functions to be the same
+	traverseCommands(rootCmd, func(cmd *cobra.Command) {
+		cmd.ValidArgsFunction = validArgs
+	})
+
 	cobra.OnInitialize(initConfig)
 
 }
 
+// Traverse the tree of commands running fn on each
+//
+// I was surprised there wasn't a cobra command to do this
+func traverseCommands(cmd *cobra.Command, fn func(*cobra.Command)) {
+	fn(cmd)
+	for _, childCmd := range cmd.Commands() {
+		traverseCommands(childCmd, fn)
+	}
+}
+
 var usageTemplate = `Usage:{{if .Runnable}}
   {{.UseLine}}{{end}}{{if .HasAvailableSubCommands}}
   {{.CommandPath}} [command]{{end}}{{if gt (len .Aliases) 0}}