patch 8.2.2397: Vim9: "%%" not seen as alternate file name for ":bdel"

Problem:    Vim9: "%%" not seen as alternate file name for commands with a
            buffer name argument.
Solution:   Recognize "%%" like "#". (closes #7732)

src/buffer.c

@@ -2564,12 +2564,15 @@ buflist_findpat(
     char_u	*p;
     int		toggledollar;
 
-    if (pattern_end == pattern + 1 && (*pattern == '%' || *pattern == '#'))
+    // "%" is current file, "%%" or "#" is alternate file
+    if ((pattern_end == pattern + 1 && (*pattern == '%' || *pattern == '#'))
+	    || (in_vim9script() && pattern_end == pattern + 2
+				    && pattern[0] == '%' && pattern[1] == '%'))
     {
-	if (*pattern == '%')
+	if (*pattern == '#' || pattern_end == pattern + 2)
-	    match = curbuf->b_fnum;
-	else
 	    match = curwin->w_alt_fnum;
+	else
+	    match = curbuf->b_fnum;
 #ifdef FEAT_DIFF
 	if (diffmode && !diff_mode_buf(buflist_findnr(match)))
 	    match = -1;

src/testdir/test_vim9_cmd.vim

@@ -68,6 +68,17 @@ def Test_expand_alternate_file()
     edit Xfiletwo
     edit %%:r
     assert_equal('Xfileone', bufname())
+
+    assert_false(bufexists('altfoo'))
+    edit altfoo
+    edit bar
+    assert_true(bufexists('altfoo'))
+    assert_true(buflisted('altfoo'))
+    bdel %%
+    assert_true(bufexists('altfoo'))
+    assert_false(buflisted('altfoo'))
+    bwipe! altfoo
+    bwipe! bar
   END
   CheckDefAndScriptSuccess(lines)
 enddef

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2397,
 /**/
     2396,
 /**/