fix(_comp_compgen): prevent IFS from affecting -- "$cur"

The IFS separators specified by the options `-s SEP` or `-l` option is
intended to take effect in evaluating `compgen`, such as the word
splitting in `-W text`.

However, if we set IFS in a separate command from `compgen`, the
arguments specified to `compgen` would also be unexpectedly affected.
Here, ${_cur:+-- "$_cur"} is intended to be expanded to two words if
`_cur` is non-empty.  However, when the separators do not contain a
whitespace, it unexpectedly results in a single word.

IFS can be specified as a temporary environment of the `compgen`
command so that it does not affect the arguments of `compgen`.

Author: akinomyoga

bash_completion

@@ -533,8 +533,7 @@ _comp_compgen()
 
     local _result
     _result=$(
-        IFS=$_ifs
-        compgen "$@" ${_cur:+-- "$_cur"}
+        IFS=$_ifs compgen "$@" ${_cur:+-- "$_cur"}
     ) || {
         local _status=$?
         if [[ $_append ]]; then