[PATCH 1 of 2 V9] update: fix bare update to work on new branch (BC)

[timeless]

This seems to have broken a number of tests:
http://buildbot.mercurial-scm.org/builders/hg%20tests/builds/196/steps/run-tests.py%20%28python%202.7.10%29/logs/stdio

Failed test-convert-mtn.t: output changed
Failed test-update-branches.t: output changed
Failed test-convert-bzr.t: output changed
Failed test-convert-bzr-114.t: output changed