tests/helper-runtests.sh

bundle: extract _processchangegroup() method

The new method applies the changegroup and fills in op.records,
sharing a little bit of code between the two callers. We'll add
another caller soon.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL