nisse-review: a commit diff Gitea can't give falls back to the whole diff

From nisse-review's review of #156: only a failed listing fell back to
the PR's whole diff; one commit's diff failing (a force-push dropped it)
failed the run. Tests also pin down its other three points, which the
code already handled: a command is taken in once, a second review at the
same head has its own marker, and `review` beside a comment is one fresh
review.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J5Utyfzw7gF7UhjRDEtjTk

nisse-review/src/nisse_review/review.py

@@ -384,23 +384,24 @@ class Reviewer:
 
     def _changes(self, repo: str, pr: PullRequest, since: str) -> str:
         """The commits since ``since``, each with its diff; the whole PR's
-        diff if Gitea can't list them (``since`` gone, say)."""
+        diff if Gitea can't list them or give one's diff (``since`` gone
+        after a force-push, say)."""
         try:
             commits = self._gitea.commits_between(repo, since, pr.head_sha)
+            diffs = "\n".join(
+                f"commit {line}\n{self._gitea.commit_diff(repo, line.split()[0])}"
+                for line in commits
+            )
         except GiteaError as exc:
             self._oplog.emit("review.compare_failed", cause=str(exc)[:300])
             commits = []
         if not commits:
             return (
-                "Gitea couldn't list the commits since then (a force-push?): "
+                "Gitea couldn't give the commits since then (a force-push?): "
                 "the whole PR's diff instead.\n"
                 + self._cut(self._gitea.pull_request_diff(repo, pr.number))
                 + "\n\n"
             )
-        diffs = "\n".join(
-            f"commit {line}\n{self._gitea.commit_diff(repo, line.split()[0])}"
-            for line in commits
-        )
         return (
             "The commits since:\n"
             + "\n".join(commits)