rust: make RevlogError AmbiguousPrefix case contain the actual prefix...
rust: make RevlogError AmbiguousPrefix case contain the actual prefix
This brings the work started in `652149ed64f0` to its logical conclusion and
makes the RevlogError self-sufficient so it can be directly converted to
CommandError, without an extra rev text annotation.
Without this change, it's confusing that the extra annotation is ignored in
most-but-not-all cases.
