__fish_complete_subcommand: Just complete -C for a given commandline

Fixes #10980.

This would, if a commandline was given, still revert to checking
the *real* commandline if it was empty.

Unfortunately, in those cases, it could have found a command and tried
to complete it.

If a commandline is given, that is what needs to be completed.

(note this means this is basically useless in completions that use it
like `sudo` and could just be replaced with `complete -C"$commandline"`)

(cherry picked from commit d5efef1cc5)

share/functions/__fish_complete_subcommand.fish

@@ -10,14 +10,12 @@ function __fish_complete_subcommand -d "Complete subcommand" --no-scope-shadowin
             case '--fcs-skip=*'
                 set skip_next (string split = -- $arg)[2]
             case --commandline # --commandline means to use our arguments instead of the commandline.
-                set subcommand $argv
-                set -e argv
-                break
+                complete -C "$argv"
+                return
         end
     end
     set -l options_with_param $argv
 
-    if not string length -q -- $subcommand
     set -l cmd (commandline -cxp | string escape) (commandline -ct)
     while set -q cmd[1]
         set -l token $cmd[1]
@@ -39,7 +37,6 @@ function __fish_complete_subcommand -d "Complete subcommand" --no-scope-shadowin
                 break
         end
     end
-    end
 
     printf "%s\n" (complete -C "$subcommand")
 end