updaterepo: add deprecation warning
We just moved all in-tree callers off of the function, but we're
leaving it here for a bit to help extension writers.
Differential Revision: https://phab.mercurial-scm.org/D9110
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL