Fix PR description filled with commit body when multiple commits exist (#8635)

* Initial plan

* Fix PR description to match GitHub.com behavior for multiple commits

When creating a PR with multiple commits, GitHub.com leaves the
description empty but the extension was filling it with the last
commit's body. Only use the commit body in the description when
there is a single commit (useBranchName is false).

Agent-Logs-Url: https://github.com/microsoft/vscode-pull-request-github/sessions/a47447af-78f0-4107-a46c-7750c8449c28

Co-authored-by: alexr00 <38270282+alexr00@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: alexr00 <38270282+alexr00@users.noreply.github.com>

Author: Copilot

src/github/createPRViewProvider.ts

@@ -809,11 +809,12 @@ export class CreatePullRequestViewProvider extends BaseCreatePullRequestViewProv
 			}
 
 			// Set description
-			if (pullRequestTemplate && lastCommit?.body) {
+			// Match GitHub.com behavior: only use the commit body when there is a single commit.
+			if (pullRequestTemplate && lastCommit?.body && !useBranchName) {
 				description = `${lastCommit.body}\n\n${pullRequestTemplate}`;
 			} else if (pullRequestTemplate) {
 				description = pullRequestTemplate;
-			} else if (lastCommit?.body && (this._pullRequestDefaults.base !== compareBranch.name)) {
+			} else if (lastCommit?.body && !useBranchName && (this._pullRequestDefaults.base !== compareBranch.name)) {
 				description = lastCommit.body;
 			}