transaction: clarify the logic around pre-finalize/post-finalize
I am taking a bit more verbose route, but I find it easier to follow for people
who (re)discover the code.
(This is a gratuitous cleanup I did while looking at something else.)
Differential Revision: https://phab.mercurial-scm.org/D8176
Hello, Foobar
[b'Hello, Fo, =;:<o', b'Hello, Bar']