rust-index: remove one collect when converting back
Turns out this is slightly faster. Sending the results back to Python is still
the most costly (like 75% of the time) of the whole method, but it's about
as fast as it can be now.
hg perf::phases on mozilla-try-2023-03-22
before: 0.267114
after: 0.247101
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL