Don't surface merge-base enrichment failures as error popups

[Copilot]

Opening certain PRs (notably cross-fork PRs with divergent histories, or where a fork has been deleted) shows an intrusive popup even though the underlying data is non-fatal enrichment:

Fetching Pull Request merge base failed: Not Found - https://docs.github.com/rest/commits/commits#compare-two-commits

FolderRepositoryManager.fulfillPullRequestMissingInfo calls repos.compareCommits purely to populate pullRequest.mergeBase. A 404 from that endpoint is a legitimate outcome for some cross-repo PRs, but the previous catch block surfaced it via vscode.window.showErrorMessage.

Changes (src/github/folderRepositoryManager.ts)

• Isolate the compareCommits failure. Wrap just that call in its own try/catch, log via Logger.warn, and fall back to pullRequest.base.sha for mergeBase — the same fallback already used in pullRequestModel.getFileChangesInfo and the Server Error branch of compareCommits in loggingOctokit.ts.
• Downgrade the outer catch from showErrorMessage to Logger.error. The whole method is best-effort enrichment; failures shouldn't block or distract from the PR view.

if (!pullRequest.mergeBase) {
    try {
        const { data } = await octokit.call(octokit.api.repos.compareCommits, { /* ... */ });
        pullRequest.mergeBase = data.merge_base_commit.sha;
    } catch (e) {
        Logger.warn(`Fetching Pull Request merge base failed: ${formatError(e)}`, this.id);
        pullRequest.mergeBase = pullRequest.base.sha;
    }
}