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
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL