sha1_file: avoid overrunning alternate object base string

While checking if a new alternate object database is a duplicate make
sure that old and new base paths have the same length before comparing
them with memcmp.  This avoids overrunning the buffer of the existing
entry if the new one is longer and it stops rejecting foobar/ after
foo/ was already added.

Signed-off-by: Rene Scharfe <ls.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -303,7 +303,8 @@ static int link_alt_odb_entry(const char *entry, const char *relative_base, int
 	 * thing twice, or object directory itself.
 	 */
 	for (alt = alt_odb_list; alt; alt = alt->next) {
-		if (!memcmp(ent->base, alt->base, pfxlen)) {
+		if (pfxlen == alt->name - alt->base - 1 &&
+		    !memcmp(ent->base, alt->base, pfxlen)) {
 			free(ent);
 			return -1;
 		}