transaction: always generate file on close

The conditionnal was buggy and file were only generated if "onclose" was
defined. By luck, "onclose" was always defined.
transaction: allow registering a temporary transaction file

During the transaction, files may be created to store or expose data
involved in the transaction (eg: changelog index data are written in
a 'changelog.i.a' for hooks). But we do not have an official way to
record such file creation and make sure they are cleaned up. The lack
of clean-up is currently okay because there is a single file involved
and a single producer/consumer.

However, as we want to expose more data (bookmarks, phases, obsmarker)
we need something more solid. The 'backupentries' mechanism could
handle that. Temporary files can be encoded as a backup of nothing
'('', <temporarypath>)'. We "need" to attach it to the same mechanism
as we use to be able to use temporary transaction files outside of
.'store/' and 'backupentries' is expected to gain such feature.

This changeset makes it clear that we should rename 'backupentries' to
something more generic.
changelog: register changelog.i.a as a temporary file

The file is registered to make sure the transaction is cleaned up in all cases.