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>

resolve-undo.c

@@ -90,7 +90,7 @@ struct string_list *resolve_undo_read(const char *data, unsigned long size)
 				continue;
 			if (size < rawsz)
 				goto error;
-			memcpy(ui->oid[i].hash, (const unsigned char *)data, rawsz);
+			oidread(&ui->oid[i], (const unsigned char *)data);
 			size -= rawsz;
 			data += rawsz;
 		}