filemerge: fix crash when using filesets in [partial-merge-tools]
Without this patch, you'd get `mercurial.error.ProgrammingError: fileset
expression with no context`.
## Avoid interference from actual test env:unset HGTEST_JOBSunset HGTEST_TIMEOUTunset HGTEST_PORTunset HGTEST_SHELL