index: make ChangeIdIndex methods fallible

[bts]

Context

This is part of a series of changes to make most methods on index traits (i.e. ChangeIdIndex, MutableIndex, ReadonlyIndex, Index) fallible. The changes will enable networked implementations of these traits, which can produce I/O errors during operation. See #7825 for more information.

This is a follow-up to #7799, which mainly reorganized error types before converting methods to return Result.

Changes

These changes convert two methods on the trait ChangeIdIndex to be fallible:

• ChangeIdIndex::resolve_prefix
• ChangeIdIndex::shortest_unique_prefix_len

Note that rather than widen the error type of rewrite::find_duplicate_divergent_commits from BackendError to some new error enum that "properly" subsumes BackendError and IndexError, I opted to add another instance of the existing pattern:

// TODO: indexing error shouldn't be a "BackendError"
.map_err(|err| BackendError::Other(err.into()))?

I figured that we already need to address some of these existing BackendError::Other(IndexError) cases, and it'll be better to tackle this sort of error reorganization separate from these fallibility changes.

Please let me know if there's anything I can fix up.

Thanks!
Brian

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added/updated tests to cover my changes

[martinvonz]

LGTM but I'll leave it to someone else to approve it since this is quite specific to things that Google and ERSC care about so I sense a bit of a conflict of interest (even though I know Yuya and Philip have already seen basically these changes).