tests/test-issue522.t

rust-hg-core: move from `ouroboros` to `self_cell`

`ouroboros` has a fundamental soundness problem that, while not applicable
today, could become applicable given new compiler optimizations.¹

`self_cell` is a crate that accomplishes a lot of the same things that
`ouroboros` did while remaining sound (that is, unless a new soundness issue
is discovered) by not assuming as much about the memory layout of the program.
`self_cell` has been scrutinized heavily in the past few months by very
competent people, some from the compiler team and has shown no weaknesses
for a while, with a 1.0 stable release coming out a couple months ago.

Our internal API is exactly the same, this is just an implementation detail.
To reiterate, no actual soundness issue was found with our use of `ouroboros`,
but there might be evolutions of `rustc` (or even a future separate compiler)
that could generate unsound code.

[1] https://github.com/joshua-maros/ouroboros/issues/88

https://bz.mercurial-scm.org/522

In the merge below, the file "foo" has the same contents in both
parents, but if we look at the file-level history, we'll notice that
the version in p1 is an ancestor of the version in p2. This test makes
sure that we'll use the version from p2 in the manifest of the merge
revision.

  $ hg init repo
  $ cd repo

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

  $ echo bar >> foo
  $ hg ci -m 'change foo'

  $ hg backout -r tip -m 'backout changed foo'
  reverting foo
  changeset 2:4d9e78aaceee backs out changeset 1:b515023e500e

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

  $ touch bar
  $ hg ci -qAm 'add bar'

  $ hg merge --debug
  resolving manifests
   branchmerge: True, force: False, partial: False
   ancestor: bbd179dfa0a7, local: 71766447bdbb+, remote: 4d9e78aaceee
   foo: remote is newer -> g
  getting foo
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)

  $ hg debugstate | grep foo
  m   0         -2 unset               foo

  $ hg st -A foo
  M foo

  $ hg ci -m 'merge'

  $ hg manifest --debug | grep foo
  c6fc755d7e68f49f880599da29f15add41f42f5a 644   foo

  $ hg debugindex foo
     rev linkrev       nodeid    p1-nodeid    p2-nodeid
       0       0 2ed2a3912a0b 000000000000 000000000000
       1       1 6f4310b00b9a 2ed2a3912a0b 000000000000
       2       2 c6fc755d7e68 6f4310b00b9a 000000000000