tests/test-manifest.t

scmutil: move construction of instability count message to separate fn

When the commad we are running, introduces new instabilities, we show a message
like `5 new orphan changesets`, `2 new content-divergent changesets`, `1 new
phase-divergent changesets` etc which is very nice.

Now taking a step ahead, we want users to show how to fix them too. Something
like:

`5 new orphan changesets (run 'hg evolve' to resolve/stabilize them)`
`2 new content-divergent changesets (run 'hg evolve --content-divergent' to
resolve them)`

and maybe telling user a way to understand more about those new instabilities
like `hg evolve --list` or `hg log -r 'orphan()'` something like that.

The idea came from issue5855 which I want to fix because fixing that will result
in a nice UI.

Taking the construction logic out will allow extensions like evolve (maybe
rebase too) to wrap that and add information about how to resolve and how to
understand the instability more.

Differential Revision: https://phab.mercurial-scm.org/D3734

Source bundle was generated with the following script:

# hg init
# echo a > a
# ln -s a l
# hg ci -Ama -d'0 0'
# mkdir b
# echo a > b/a
# chmod +x b/a
# hg ci -Amb -d'1 0'

  $ hg init
  $ hg unbundle "$TESTDIR/bundles/test-manifest.hg"
  adding changesets
  adding manifests
  adding file changes
  added 2 changesets with 3 changes to 3 files
  new changesets b73562a03cfe:5bdc995175ba
  (run 'hg update' to get a working copy)

The next call is expected to return nothing:

  $ hg manifest

  $ hg co
  3 files updated, 0 files merged, 0 files removed, 0 files unresolved

  $ hg manifest
  a
  b/a
  l

  $ hg files -vr .
           2   a
           2 x b/a
           1 l l
  $ hg files -r . -X b
  a
  l

  $ hg manifest -v
  644   a
  755 * b/a
  644 @ l

  $ hg manifest --debug
  b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3 644   a
  b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3 755 * b/a
  047b75c6d7a3ef6a2243bd0e99f94f6ea6683597 644 @ l

  $ hg manifest -r 0
  a
  l

  $ hg manifest -r 1
  a
  b/a
  l

  $ hg manifest -r tip
  a
  b/a
  l

  $ hg manifest tip
  a
  b/a
  l

  $ hg manifest --all
  a
  b/a
  l

The next two calls are expected to abort:

  $ hg manifest -r 2
  abort: unknown revision '2'!
  [255]

  $ hg manifest -r tip tip
  abort: please specify just one revision
  [255]