fix(_comp_delimited): treat delimiter as a fixed string

The right-hand side of == in the conditional commands [[ ]] are
treated as a glob pattern.  In the other occurrences of $delimiter,
they are quoted as "$delimiter", which means that $delimiter treated
as a fixed string.  However, only in this line, $delimiter is treated
as a glob pattern.  In this patch, we quote it properly the same as in
the other occurrences of $delimiter.

Author: akinomyoga

bash_completion

@@ -1199,7 +1199,7 @@ _comp_delimited()
         delimiter=$1
         shift
     fi
-    [[ $cur == *$delimiter* ]] && prefix=${cur%"$delimiter"*}$delimiter
+    [[ $cur == *"$delimiter"* ]] && prefix=${cur%"$delimiter"*}$delimiter
 
     if [[ $deduplicate ]]; then
         # We could construct a -X pattern to feed to compgen, but that'd