write: add the possibility to pass keyword argument from batchget to vfs
We are going to pass atomictemp keyword argument from merge.baychget to
vfs.__call__. Update all the frames to accept **kwargs and pass it to the next
function.
Differential Revision: https://phab.mercurial-scm.org/D1881
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL