match: make _fileroots a @propertycache and rename it to _fileset
The files in the set are not necesserily roots of anything. Making it
a @propertycache will help towards extracting a base class for
matchers.
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL