exclude_existing(): set existing_refs.strdup_strings

The each_ref_fn add_existing() adds refnames to the existing_refs
list.  But the lifetimes of these refnames is not guaranteed by the
refs API, so configure the string_list to make copies as it adds them.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/show-ref.c

@@ -103,7 +103,7 @@ static int add_existing(const char *refname, const unsigned char *sha1, int flag
  */
 static int exclude_existing(const char *match)
 {
-	static struct string_list existing_refs = STRING_LIST_INIT_NODUP;
+	static struct string_list existing_refs = STRING_LIST_INIT_DUP;
 	char buf[1024];
 	int matchlen = match ? strlen(match) : 0;