transaction: allow finalizer to add finalizer
It will make some code (persistent nodemap related) simpler to write, because
higher level code can blindly queue finalization without thinking too hard about
the context.
Differential Revision: https://phab.mercurial-scm.org/D7833
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown