Support review comments in the per-commit diff view

[daandemeyer]

Until now, opening a file diff from the commits tree of a PR showed no commenting UI and no existing review comments — commenting only worked from the changed-files tree. This change adds full commenting support to the per-commit diff view, anchoring new comments to the commit being viewed.

Display side

• Add a fourth thread map _commitFileChangeCommentThreads, keyed by ${path}@${originalCommitId} so multiple commits' threads on the same file don't collide.
• For every non-outdated thread with a known originalCommitId, mirror it on a per-commit review://...?commit=...&isOutdated=true URI that matches the one CommitNode produces, in addition to its existing workspace/review-scheme entry.
• Consolidate createOutdatedCommentThread with the new commit-scoped variant into a single createCommitAnchoredCommentThread (the URI shape is identical — only the call sites differ).
• Replace _findMatchingThread with _findMatchingThreads / _removeMatchingThreads so changed/removed thread events update or dispose both the workspace/review mirror and the commit-scoped one.
• provideCommentingRanges falls back to fetching the commit's patch via a new PullRequestModel.getCommitFileDiffHunks helper when findMatchedFileChangeForReviewDiffView returns no match (because commit-scoped URIs aren't in localFileChanges, which only tracks the PR head).
• Wire the new map into onDidChangePendingReviewState and simplify updateCommentExpandState to iterate all four maps via a shared helper.

Write side

• PullRequestModel.createReviewThread takes a new optional commitId parameter. When commitId !== HEAD, the model routes through a new private createReviewThreadOnCommit helper that uses the (deprecated but still-functional) addPullRequestReviewComment GraphQL mutation with explicit commitOID. This is necessary because addPullRequestReviewThread always anchors new threads to the PR head regardless of which commit the pending review was created on, so the deprecated path is the only way to anchor a new comment to a non-head commit. After the mutation, the threads cache is refreshed via getReviewThreads() so the new thread propagates through the existing onDidChangeReviewThreads pipeline.
• The deprecated mutation only accepts a single integer position (the diff offset from the first hunk header), not line/side ranges, so a new computeDiffPositionForLine helper walks the commit's diff hunks to map a line number to a position. Multi-line ranges collapse to a single line at endLine in this fallback path.
• New controller helpers getFileNameForThread, getCommitForThread and getCommentLinesForThread extract path, commit and line numbers from any thread URI. Crucially, for commit-scoped review URIs the filename comes from query.path rather than gitRelativeRootPath(uri.path), since CommitNode builds URIs via reviewPath() which produces a synthetic commit~ec597364/foo/bar prefix that nodePath.relative turns into garbage. Editor line numbers for review-scheme URIs are used directly (no mapping through workspace diffs), since they already match the file at the ref being viewed.
• startReview (the controller method) and createOrReplyComment now forward getCommitForThread(thread) for both new threads and replies, so replying to a comment in a non-head commit's diff lands on the right commit too.

[daandemeyer]

@microsoft-github-policy-service agree