paycheck: src/dst -> old/new in applier code

The old/new terminology is consistent with that used in update payload
manifest, as well as other parts of the update_payload library (e.g.
checker).

BUG=None
TEST=Unit/integration test passes

Change-Id: I91244ae8adf073b958e8cd7e7670341f056c848e
Reviewed-on: https://gerrit.chromium.org/gerrit/50130
Tested-by: Gilad Arnold <garnold@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
diff --git a/scripts/paycheck.py b/scripts/paycheck.py
index 0ca2a76..42fade6 100755
--- a/scripts/paycheck.py
+++ b/scripts/paycheck.py
@@ -207,8 +207,8 @@
           payload.Apply(extra_args[0], extra_args[1])
         elif options.assert_type == _TYPE_DELTA:
           payload.Apply(extra_args[0], extra_args[1],
-                        src_kernel_part=extra_args[2],
-                        src_rootfs_part=extra_args[3])
+                        old_kernel_part=extra_args[2],
+                        old_rootfs_part=extra_args[3])
         else:
           assert False, 'cannot get here'