remotefilelog: tell runbgcommand to not block on child process startup
These two invocations will always find a binary because they're
re-running hg. As a result, we can skip waiting for the subprocess to
start running and save a little bit of wall-time.
Differential Revision: https://phab.mercurial-scm.org/D6539
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL