mailmap: resurrect lower-casing of email addresses

Commit 0925ce4(Add map_user() and clear_mailmap() to mailmap) broke the
lower-casing of email addresses.  This mostly did not matter if your
.mailmap has only lower-case email addresses;  However, we did not
require .mailmap to contain lowercase-only email addresses.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mailmap.c

@@ -50,6 +50,15 @@ static void add_mapping(struct string_list *map,
 {
 	struct mailmap_entry *me;
 	int index;
+	char *p;
+
+	if (old_email)
+		for (p = old_email; *p; p++)
+			*p = tolower(*p);
+	if (new_email)
+		for (p = new_email; *p; p++)
+			*p = tolower(*p);
+
 	if (old_email == NULL) {
 		old_email = new_email;
 		new_email = NULL;