feat(__load_completion): localize default $IFS

In order to be able to wrap loading 3rd party completions in
enable+reset posix mode.

#904 (comment)

Author: scop

bash_completion

@@ -2537,6 +2537,9 @@ __load_completion()
         backslash=\\
     fi
 
+    # For loading 3rd party completions wrapped in shopt reset
+    local IFS=$' \t\n'
+
     for dir in "${dirs[@]}"; do
         [[ -d $dir ]] || continue
         for compfile in "$cmd" "$cmd.bash"; do