assert: reduce diff noise

Assertion errors that produce a diff show a diff for identical entries
in case one side of the comparison has more object properties than the
other one. Those lines are now taken into account and will not show up
as diverging lines anymore.

Refs: https://github.com/nodejs/node/issues/22763

PR-URL: https://github.com/nodejs/node/pull/23048
Refs: https://github.com/nodejs/node/issues/22763


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>