Convert struct object to object_id

struct object is one of the major data structures dealing with object
IDs.  Convert it to use struct object_id instead of an unsigned char
array.  Convert get_object_hash to refer to the new member as well.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Jeff King <peff@peff.net>

builtin/worktree.c

@@ -277,7 +277,7 @@ static int add_worktree(const char *path, const char *refname,
 
 	if (commit)
 		argv_array_pushl(&cp.args, "update-ref", "HEAD",
-				 sha1_to_hex(commit->object.sha1), NULL);
+				 oid_to_hex(&commit->object.oid), NULL);
 	else
 		argv_array_pushl(&cp.args, "symbolic-ref", "HEAD",
 				 symref.buf, NULL);