Complete tar with .xbps files.

Backported from upstream https://github.com/scop/bash-completion/pull/688

--- a/completions/tar
+++ b/completions/tar	2022-01-23 11:42:19.699734913 -0300
@@ -422,7 +422,7 @@
 
 __tar_detect_ext()
 {
-    local tars='@(@(tar|gem|spkg)?(.@(Z|[bgx]z|bz2|lz?(ma|o)|zst))|t@([abglx]z|b?(z)2|zst))'
+    local tars='@(@(tar|spkg)?(.@(Z|[bgx]z|bz2|lz?(ma|o)|zst))|t@([abglx]z|b?(z)2|zst)|gem|xbps)'
     ext="$tars"
 
     case "$tar_mode_arg" in
@@ -430,7 +430,7 @@
             # Should never happen?
             ;;
         ?(-)*[cr]*f)
-            ext='@(tar|gem|spkg)'
+            ext='@(tar|gem|spkg|xbps)'
             case ${words[1]} in
                 *a*) ext="$tars" ;;
                 *z*) ext='t?(ar.)gz' ;;