Fix completions for service

I'm not sure what was up with the old completions,
`$__fish_service_commands` is not set anywhere and completions for the
command (not the service) were not being generated on my machine.

CHANGELOG.md

@@ -80,6 +80,7 @@ This section is for changes merged to the `major` branch that are not also merge
   - `git` (#4395, #4396, #4592)
   - `npm`<sup>†</sup>
   - `python`/`python2`/`python3`
+  - `service`
   - `ssh` (#4344)
   - `sudo` (see above)
   - `yarn`<sup>&#x2217;</sup><sup>†</sup>

share/completions/service.fish

@@ -1,6 +1,19 @@
+set -l service_commands
+
+# as found in __fish_print_service_names.fish
+if test -d /run/systemd/system # Systemd systems
+	set service_commands start stop restart status enable disable
+else if type -f rc-service 2>/dev/null # OpenRC (Gentoo)
+	set service_commands start stop restart
+else if test -d /etc/init.d # SysV on Debian and other linuxen
+	set service_commands start stop "--full-restart"
+else # FreeBSD
+	set service_commands start stop start_once stop_once
+end
+
 # Fist argument is the names of the service, i.e. a file in /etc/init.d
-complete -c service -n "test (count (commandline -poc)) = 1" -xa "(__fish_print_service_names)" -d "Service name"
+complete -c service -n "__fish_is_first_token" -xa "(__fish_print_service_names)" -d "Service name"
 
 #The second argument is what action to take with the service
-complete -c service -n "test (count (commandline -poc)) -gt 1" -xa '$__fish_service_commands'
+complete -c service -n "not __fish_is_first_token" -xa "$service_commands"