refactor(ssh-add): use _comp_compgen_help

We also remove the backslash of '-\?' within this patch.  The
backslash was introduced in commit 60b8fab, which seems to have tried
to work around the failglob failure.  Because we now directly specify
the arguments to _comp_compgen_help so that we do not have to evaluate
unquoted $2, we do not need to add the backslash anymore.  Note that
the backslash added to ssh-keygen in 60b8fab was removed in commit
4dc0016 where _parse_help was switched to _parse_usage.

References:

60b8fab
4dc0016

Author: akinomyoga

completions/ssh-add

@@ -24,7 +24,7 @@ _comp_cmd_ssh_add()
     esac
 
     if [[ $cur == -* ]]; then
-        COMPREPLY=($(compgen -W '$(_parse_help "$1" "-\?")' -- "$cur"))
+        _comp_compgen_help -- '-?'
         return
     fi