diff-delta.c: Fix broken skip calculation.

A particularly bad case was HASH_LIMIT <= hash_count[i] < 2*HASH_LIMIT:
in that case, only a single hash survived.  For larger cases,
2*HASH_LIMIT was the actual limiting value after pruning.

Signed-off-by: David Kastrup <dak@gnu.org>
Acked-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff-delta.c

@@ -213,7 +213,7 @@ struct delta_index * create_delta_index(const void *buf, unsigned long bufsize)
 		entry = hash[i];
 		do {
 			struct index_entry *keep = entry;
-			int skip = hash_count[i] / HASH_LIMIT / 2;
+			int skip = hash_count[i] / HASH_LIMIT;
 			do {
 				entry = entry->next;
 			} while(--skip && entry);