_filedir*: update link to bug-bash discussion on -X (#404)

Replace an unreachable gmane.org reference with a link to the GNU mailing list
archive.  Also provide an alternative URL including the message-id that should
make it easier to find the cited email somewhere else in case the GNU link
becomes dead too.

bash_completion

@@ -569,7 +569,8 @@ _filedir()
         _quote_readline_by_ref "$cur" quoted
 
         # Munge xspec to contain uppercase version too
-        # http://thread.gmane.org/gmane.comp.shells.bash.bugs/15294/focus=15306
+        # https://lists.gnu.org/archive/html/bug-bash/2010-09/msg00036.html
+        # news://news.gmane.io/4C940E1C.1010304@case.edu
         local xspec=${1:+"!*.@($1|${1^^})"} plusdirs=()
 
         # Use plusdirs to get dir completions if we have a xspec; if we don't,
@@ -1982,7 +1983,8 @@ _filedir_xspec()
         ))
 
     # Munge xspec to contain uppercase version too
-    # http://thread.gmane.org/gmane.comp.shells.bash.bugs/15294/focus=15306
+    # https://lists.gnu.org/archive/html/bug-bash/2010-09/msg00036.html
+    # news://news.gmane.io/4C940E1C.1010304@case.edu
     eval xspec="${xspec}"
     local matchop=!
     if [[ $xspec == !* ]]; then