tests/test-merge-subrepos.t

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.

  $ hg init

  $ echo a > a
  $ hg ci -qAm 'add a'

  $ hg init subrepo
  $ echo 'subrepo = http://example.net/libfoo' > .hgsub
  $ hg ci -qAm 'added subrepo'

  $ hg up -qC 0
  $ echo ax > a
  $ hg ci -m 'changed a'
  created new head

  $ hg up -qC 1
  $ cd subrepo
  $ echo b > b
  $ hg add b
  $ cd ..

Should fail, since there are added files to subrepo:

  $ hg merge
  abort: uncommitted changes in subrepository 'subrepo'
  [255]