tests/test-gendoc-pt_BR.t

rebase: change "result would have 3 parent" error message (BC)

The old error message "cannot use revision REV as base, result would have 3
parents" is confusing - why use REV as base? why add a new parent?.

This patch changes it to "cannot move parent", which seems better.

Differential Revision: https://phab.mercurial-scm.org/D342

#require docutils gettext

  $ $TESTDIR/check-gendoc pt_BR
  checking for parse errors