Fix new PR title to match github.com when branch has multiple commits

[Copilot]

Creating a new PR via the extension always used the last commit's title, even when the branch had multiple commits — github.com uses the branch name in that case.

CreatePullRequestViewProvider.getTitleAndDescription already intended to mirror github.com (branch name for >1 commits, commit title for 1), but its commit count came solely from the GitHub compareCommits API via getTotalGitHubCommits. That call returns undefined when the compare branch has no upstream, the head GitHub repo can't be resolved by remote name, or the API otherwise fails, at which point the code set useBranchName = false unconditionally and fell back to the tip commit's title.

Changes

• src/github/createPRViewProvider.ts: when the GitHub commit count is unavailable, fall back to a local git log via this.model.gitCommits() (already runs git log <baseRemote>/<base>..<compare>), filter out merge commits, and feed that count into the existing branch-name-vs-commit-title decision. If both sources fail, behavior is unchanged (uses tip commit). Description logic keyed off !useBranchName benefits automatically.

let totalNonMergeCommitsCount: number | undefined =
    totalCommits?.filter(c => c.parents.length < 2).length;

if (totalNonMergeCommitsCount === undefined) {
    try {
        const localCommits = await this.model.gitCommits();
        totalNonMergeCommitsCount = localCommits.filter(c => c.parents.length < 2).length;
    } catch (e) {
        Logger.debug(`Failed to retrieve local git commits as fallback for PR title for branch ${name}: ${e}`, CreatePullRequestViewProvider.ID);
    }
}