fix(__load_completion): pass full path to fallback completion loaders

Passing just the basename results in failures when a command invoked
(with a path) is not in `$PATH`.

Also facilitates correct command instance being used to generate the
completion in our fallback completion loaders. Completions generated by
different instances of the command might vary.

Author: scop

bash_completion

@@ -2576,7 +2576,7 @@ __load_completion()
             # Do not warn with . or .. (especially the former is common)
             [[ $compfile == */.?(.) ]] ||
                 echo "bash_completion: $compfile: is a directory" >&2
-        elif [[ -e $compfile ]] && . "$compfile" "$cmdname"; then
+        elif [[ -e $compfile ]] && . "$compfile" "$cmd" "$@"; then
             [[ $backslash ]] && $(complete -p "$cmdname") "\\$cmdname"
             return 0
         fi