grep: convert to struct object_id

Convert the remaining parts of grep to use struct object_id.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1026,6 +1026,13 @@ static inline void oidcpy(struct object_id *dst, const struct object_id *src)
 	hashcpy(dst->hash, src->hash);
 }
 
+static inline struct object_id *oiddup(const struct object_id *src)
+{
+	struct object_id *dst = xmalloc(sizeof(struct object_id));
+	oidcpy(dst, src);
+	return dst;
+}
+
 static inline void hashclr(unsigned char *hash)
 {
 	memset(hash, 0, GIT_SHA1_RAWSZ);