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
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif