_longopt: Run commands in C locale.

This is a follow-up to commit 822735146f

bash_completion

@@ -1781,7 +1781,7 @@ _longopt()
             return 0
             ;;
         --+([-a-z0-9_]))
-            local argtype=$( $1 --help 2>&1 | sed -ne \
+            local argtype=$( LC_ALL=C $1 --help 2>&1 | sed -ne \
                 "s|.*$prev\[\{0,1\}=[<[]\{0,1\}\([-A-Za-z0-9_]\{1,\}\).*|\1|p" )
             case ${argtype,,} in
                 *dir*)
@@ -1799,7 +1799,7 @@ _longopt()
     $split && return 0
 
     if [[ "$cur" == -* ]]; then
-        COMPREPLY=( $( compgen -W "$( $1 --help 2>&1 | \
+        COMPREPLY=( $( compgen -W "$( LC_ALL=C $1 --help 2>&1 | \
             sed -ne 's/.*\(--[-A-Za-z0-9]\{1,\}=\{0,1\}\).*/\1/p' | sort -u )" \
             -- "$cur" ) )
         [[ $COMPREPLY == *= ]] && compopt -o nospace