branch: delete obsolete message about changing branch of obsolete commit
We now rely on `rewriteutil.precheck()` to check for divergence, so we
don't need the extra check in `cmdutil.changebranch()`. The former
check is a little less strict in that it allows you to rewrite a
commit without non-obsolete successors.
Differential Revision: https://phab.mercurial-scm.org/D10519
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true