cache: add a function to read an object ID from a buffer

In various places throughout the codebase, we need to read data into a
struct object_id from a pack or other unsigned char buffer.  Add an
inline function that does this based on the current hash algorithm in
use, and use it in several places.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache-tree.c

@@ -523,7 +523,7 @@ static struct cache_tree *read_one(const char **buffer, unsigned long *size_p)
 	if (0 <= it->entry_count) {
 		if (size < rawsz)
 			goto free_return;
-		memcpy(it->oid.hash, (const unsigned char*)buf, rawsz);
+		oidread(&it->oid, (const unsigned char *)buf);
 		buf += rawsz;
 		size -= rawsz;
 	}