review: Remap drafts to remote change head

[abhinav]

Review comment drafts need a stable revision for their file and line anchors.
Record the local branch head before opening the comment editor so concurrent
branch movement cannot pair an old anchor with a newer commit.

At publication, retrieve the forge's current change head and map each root
anchor from its recorded commit to that remote revision.
Use the same remote head for review-patch validation so submitted coordinates
and local checks describe one revision.
Reuse one lazily loaded source-to-head patch for drafts that share a source
commit.

Mapping follows ordinary edits and Git-detected renames for file, line, and
range comments.
A source or diff-loading failure warns and falls back to the saved coordinate;
a target deleted without replacement remains an error.
No refs are retained solely for draft comments.

[abhinav]

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
      • reviewdiff: Stream branch diffs #1431
        • review: Separate commands from workflows #1432
          • state: Make review draft updates atomic #1438
            • state: Couple drafts to branch lifecycle #1439
              • state: Preserve drafts changed during publication #1440
                • review: Delete local drafts #1441
                  • review: Configure the draft default #1442
                  • review: Accept file and range drafts #1443
                  • review: Remap drafts to remote change head #1445 ◀

_{Change managed by git-spice.}