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
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