context: make sure file is not deleted while checking path conflicts
If a file is deleted and a directory of same name is created in the same commit,
IMM thinks of that as a file conflict, however the file is deleted and hence the
directory can be created.
The test change demonstrate the fix.
Differential Revision: https://phab.mercurial-scm.org/D4300
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL