lib id_prefix: look for divergent changes outside short prefix set

[ilyagr]

Fixes https://github.com/martinvonz/jj/issues/2476.

Previously, if there was a change id match within the short prefix lookup set, jj would not look for commits with that same change id outside the short prefix set. So, it wouldn't find the conflicted commits for a commit with a divergent (AKA conflicted) change id.

Checklist

If applicable:

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

[ilyagr]

Thank you, Yuya!