revlog: always use a Rust index for REVLOGv1 if rustext is present
We are about to change classes such as `rustext.AncestorsIterator` to
take a Rust index, hence we cannot have the option not to use the Rust
index.
Note: this can be refined depending on whether we want to keep this
option or not. We will have to make two versions of `AncestorsIterator`
and its sibling to support REVLOGV2 and CHANGELOGv2 anyway.
Meanwhile, this is the simplest change to make the tests pass.
Test issue2761
$ hg init repo
$ cd repo
$ touch to-be-deleted
$ hg add
adding to-be-deleted
$ hg ci -m first
$ echo a > to-be-deleted
$ hg ci -m second
$ rm to-be-deleted
$ hg diff -r 0
Same issue, different code path
$ hg up -C
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ touch does-not-exist-in-1
$ hg add
adding does-not-exist-in-1
$ hg ci -m third
$ rm does-not-exist-in-1
$ hg diff -r 1