shamhub: Compute review thread staleness

[ed-irl]

ShamHub review threads record the change head in ReviewThread.CommitHash. When that revision differs from the current head, compare the two revisions and mark a right-side thread outdated only when the later patch deletes a line in its inclusive range.

Insertions alone do not stale an anchor. File-level and left-side threads retain their stored state because their recorded coordinates do not identify a comparable source range. Explicitly seeded outdated state still takes precedence, and diff or parsing failures propagate to the caller.

This builds on the review patch queries from #1162. The user-facing staleness script is included with the commands in #1163.

[ed-irl]

This change is part of the following stack:

• reviewdiff: Parse review comment patches #1162
  • shamhub: Compute review thread staleness #1222 ◀
    • Add review comment commands #1163
      • branch comment: Support file and line-range anchors #1223
        • branch comment: Extend review comment JSON #1224

_{Change managed by git-spice.}