tests/test-revlog-packentry.t

localrepo: stop doing special dirstate backup at transaction open

Since the dirstate writes are already managed by the transaction, we already do
a backup of the dirstate when necessary (and even trigger one to keep `hg
rollback` happy).

We needs some special code to deal with the initial empty checkout, but it is
not too complicated.

Managing variable filename (as dirstate-v2 uses) at the "journalfile" level, is
complex and fragile (which is consistent with the fact these files are not
journal…). If we no longer do it, our life is significantly simpler.

In some sense, we apply the xkcd-1134¹ solution to our savebackup/restorebackup
problem.

[1] https://xkcd.com/1134/

(the change to test-hardlink are expect as decreasing the number of duplicated
backup drive the hardlink count down)

  $ hg init repo
  $ cd repo

  $ touch foo
  $ hg ci -Am 'add foo'
  adding foo

  $ hg up -C null
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

this should be stored as a delta against rev 0

  $ echo foo bar baz > foo
  $ hg ci -Am 'add foo again'
  adding foo
  created new head

  $ hg debugindex foo
     rev linkrev       nodeid    p1-nodeid    p2-nodeid
       0       0 b80de5d13875 000000000000 000000000000
       1       1 0376abec49b8 000000000000 000000000000

  $ cd ..