indexer: don't worry about duplicate objects

If a packfile includes duplicate objects, we can choose to use the
secon copy instead of the first by using the same logic as if it were
the first.

Change the error condition from 0 to -1, which indicates a bad resize,
and set the OOM message in that case.

This does mean we will leak the first copy of the object. We can deal
with that later, but making fetches work is more important.

src/indexer.c

@@ -287,9 +287,9 @@ static int store_object(git_indexer *idx)
 	pentry->offset = entry_start;
 
 	k = kh_put(oid, idx->pack->idx_cache, &pentry->sha1, &error);
-	if (!error) {
+	if (error == -1) {
 		git__free(pentry);
-		giterr_set(GITERR_INDEXER, "cannot handle duplicate objects in pack");
+		giterr_set_oom();
 		goto on_error;
 	}