rust-index: stop instantiating a C Index
The only missing piece was the `cache` to be returned from
`revlog.parse_index_v1_mixed`, and it really seems that it is
essentially repetition of the input, if `inline` is `True`.
Not worth a Rust implementation (C implementation is probably there
for historical reasons).
# So GitLab doesn't think we're using tons of Perl
*.t -linguist-detectable