perf(filedir_xspec): avoid double negation !(!(...)) in extglob

Due to the Bash implementatin of negation matching, nested negations
becomes extremely slow for a long filename. "!(!(...))" is
interchangeable with "@(...)".

The actual overhead caused by "!(!(...))" largely depends on the
pattern specified to "...", but it is hard to predict.  For example,
"+(0)+(?).txt" or "+(+(0)).txt" may cause a large delay.  Although
those specific patterns are not used in bash-completion, but it is not
predictable whether other patterns in bash-completion cause the same
problem.  It is recommended to avoid "!(!(...))" in general.

Examples:

  $ time bash -O extglob -c 'echo !(!(+(+(0)).txt))'
  00000000000000000000.txt

  real    0m4.537s

  $ time bash -O extglob -c 'echo !(!(+(0)+(?).txt))'
  0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.txt

  real    0m0.991s

Author: akinomyoga

bash_completion

@@ -3264,14 +3264,14 @@ _comp_compgen_filedir_xspec()
     # 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 [[ $xspec == !* ]]; then
         xspec=${xspec#!}
-        matchop=@
+        xspec="@(|!($xspec|${xspec^^}))"
+    else
+        xspec="@(|$xspec|${xspec^^})"
     fi
-    xspec="$matchop($xspec|${xspec^^})"
 
-    _comp_compgen -av toks -c "$quoted" -- -f -X "@(|!($xspec))"
+    _comp_compgen -av toks -c "$quoted" -- -f -X "$xspec"
 
     # Try without filter if it failed to produce anything and configured to
     [[ ${BASH_COMPLETION_FILEDIR_FALLBACK-} && ${#toks[@]} -lt 1 ]] &&