Change-namespaced short commit IDs

[mlcui-corp]

At the moment, commit IDs (cf. change IDs) are typically used for:

• Manipulating (abandoning) divergent changes - two visible commits with the same change ID.
• Manipulating (restoring) old versions of changes, found through jj obslog, jj --at-op, or an old invocation of jj log.
• Git interoperability, for commands such as git push origin COMMIT_ID:refs/for/main. These cannot use short IDs.

The current status quo of short commit IDs - unique prefixes for all visible commits in revsets.short-prefixes, then globally-unique prefixes - is fine for the divergent change use case, but falls short for the "old versions of changes" use case:

• Short commit IDs from the output of an old jj log invocation (equivalently, jj log --at-op) often don't work, as those commits became invisible. Restoring those require a longer prefix.
• Short commit IDs for hidden commits - such as the ones in jj obslog - are very long, as they need to be globally unique prefixes.

My proposal would be to allow specification of short commit IDs in the namespace of a given change. Using a (not final) syntax of change??commit, tnpw??a8 would refer to "the unique (visible) commit in the obslog of change tnpw with prefix a8".

If such a thing existed:

• Divergent changes in jj log could be shown as change??short_visible_commit instead of the current change??. As there are commonly only two visible commits for a divergent change, short_visible_commit will often only be a single character.
• Specifying a commit from a divergent change might be more intuitive - you specify the change, then which commit to refer to.
• Commit IDs could be shown as change??short_commit (not over visible commits), so short commit IDs from old jj log outputs should still work even if the commit was hidden (as long as there are no new commits with the same change and prefix).
• jj obslog commit IDs would be much shorter.