patch 8.2.2974: Greek spell checking uses wrong case folding

Problem:    Greek spell checking uses wrong case folding.
Solution:   Fold capital sigma depending on whether it is at the end of a
            word or not. (closes #299)
diff --git a/src/spellfile.c b/src/spellfile.c
index 2018169..264264c 100644
--- a/src/spellfile.c
+++ b/src/spellfile.c
@@ -3429,9 +3429,9 @@
     if (ga_grow(gap, 1) == OK)
     {
 	ftp = ((fromto_T *)gap->ga_data) + gap->ga_len;
-	(void)spell_casefold(from, (int)STRLEN(from), word, MAXWLEN);
+	(void)spell_casefold(curwin, from, (int)STRLEN(from), word, MAXWLEN);
 	ftp->ft_from = getroom_save(spin, word);
-	(void)spell_casefold(to, (int)STRLEN(to), word, MAXWLEN);
+	(void)spell_casefold(curwin, to, (int)STRLEN(to), word, MAXWLEN);
 	ftp->ft_to = getroom_save(spin, word);
 	++gap->ga_len;
     }
@@ -4391,7 +4391,7 @@
     int		res = OK;
     char_u	*p;
 
-    (void)spell_casefold(word, len, foldword, MAXWLEN);
+    (void)spell_casefold(curwin, word, len, foldword, MAXWLEN);
     for (p = pfxlist; res == OK; ++p)
     {
 	if (!need_affix || (p != NULL && *p != NUL))