__fish_complete_command: Fix --foo= logic

This was already supposed to handle `--foo=bar<TAB>` cases, except it
printed the `--foo=` again, causing fish to take that as part of the
token.

See #9538 for a similar thing with __fish_complete_directories.

Fixes #10011

(cherry picked from commit b03327f5d2)

share/functions/__fish_complete_command.fish

@@ -3,7 +3,7 @@ function __fish_complete_command --description 'Complete using all available com
     switch $ctoken
         case '*=*'
             set ctoken (string split "=" -- $ctoken)
-            printf '%s\n' $ctoken[1]=(complete -C "$ctoken[2]")
+            complete -C "$ctoken[2]"
         case '-*' # do not try to complete options as commands
             return
         case '*'

tests/checks/complete.fish

@@ -532,3 +532,7 @@ begin
 end
 
 rm -r $tmpdir
+
+complete -C'complete --command=fish' | head -n 1 | string replace -rf '\t.*' ''
+# (one "--command=" is okay, we used to get "--command=--command="
+# CHECK: --command=fish