rewriteutil: say how many commits would become orphan if commit is rewritten
This copies the message from the evolve extension, but modifies it a
bit to work with the grammar (in particular with the use of "change
branch of" as action). I don't know why it doesn't use the same
_formatrevs() as for public commmits.
Differential Revision: https://phab.mercurial-scm.org/D10671