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.
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https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
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273ce12ad8f1` will report some style violations on a very old commit).