tests/test-context-metadata.t

phases: invalidate the phases set less often on retract boundary

We already have the information to update the phase set, so we do so directly
instead of invalidating the cache.

This show a sizeable speedup in our `perf::unbundle` benchmark on the
many-draft mozilla-try repository.

### data-env-vars.name            = mozilla-try-2023-03-22-zstd-sparse-revlog
  # benchmark.name                = hg.perf.perf-unbundle
  # bin-env-vars.hg.flavor        = no-rust
  # bin-env-vars.hg.py-re2-module = default
  # benchmark.variants.issue6528  = disabled

  # benchmark.variants.revs = last-10
before: 2.055259 seconds
after:  1.887064 seconds (-8.18%)
  # benchmark.variants.revs = last-100
before: 2.409239 seconds
after:  2.222429 seconds (-7.75%)
  # benchmark.variants.revs = last-1000
before: 3.945648 seconds
after:  3.762480 seconds (-4.64%)

Tests about metadataonlyctx

  $ hg init
  $ echo A > A
  $ hg commit -A A -m 'Add A'
  $ echo B > B
  $ hg commit -A B -m 'Add B'
  $ hg rm A
  $ echo C > C
  $ echo B2 > B
  $ hg add C -q
  $ hg commit -m 'Remove A'

  $ cat > metaedit.py <<EOF
  > from mercurial import context, pycompat, registrar
  > cmdtable = {}
  > command = registrar.command(cmdtable)
  > @command(b'metaedit')
  > def metaedit(ui, repo, arg):
  >     # Modify commit message to "FOO"
  >     with repo.wlock(), repo.lock(), repo.transaction(b'metaedit'):
  >         old = repo[b'.']
  >         kwargs = dict(s.split(b'=', 1) for s in arg.split(b';'))
  >         if b'parents' in kwargs:
  >             kwargs[b'parents'] = map(int, kwargs[b'parents'].split(b','))
  >         new = context.metadataonlyctx(repo, old,
  >                                       **pycompat.strkwargs(kwargs))
  >         new.commit()
  > EOF
  $ hg --config extensions.metaedit=$TESTTMP/metaedit.py metaedit 'text=Changed'
  $ hg log -r tip
  changeset:   3:ad83e9e00ec9
  tag:         tip
  parent:      1:3afb7afe6632
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     Changed
  
  $ hg --config extensions.metaedit=$TESTTMP/metaedit.py metaedit 'parents=0' 2>&1 | grep -E '^RuntimeError'
  RuntimeError: can't reuse the manifest: its p1 doesn't match the new ctx p1

  $ hg --config extensions.metaedit=$TESTTMP/metaedit.py metaedit 'user=foo <[e-mail]>'
  $ hg log -r tip
  changeset:   4:1f86eaeca92b
  tag:         tip
  parent:      1:3afb7afe6632
  user:        foo <[e-mail]>
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     Remove A