Fix fork PR checkout to properly set upstream tracking branch

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

Author: Copilot

src/github/pullRequestGitHelper.ts

@@ -59,15 +59,21 @@ export class PullRequestGitHelper {
 		}
 
 		// fetch the branch
-		const ref = `${pullRequest.head.ref}:${localBranchName}`;
-		Logger.debug(`Fetch ${remoteName}/${pullRequest.head.ref}:${localBranchName} - start`, PullRequestGitHelper.ID);
-		progress.report({ message: vscode.l10n.t('Fetching branch {0}', ref) });
-		await repository.fetch(remoteName, ref);
-		Logger.debug(`Fetch ${remoteName}/${pullRequest.head.ref}:${localBranchName} - done`, PullRequestGitHelper.ID);
-		progress.report({ message: vscode.l10n.t('Checking out {0}', ref) });
+		Logger.debug(`Fetch ${remoteName}/${pullRequest.head.ref} - start`, PullRequestGitHelper.ID);
+		progress.report({ message: vscode.l10n.t('Fetching branch {0}', pullRequest.head.ref) });
+		await repository.fetch(remoteName, pullRequest.head.ref);
+		Logger.debug(`Fetch ${remoteName}/${pullRequest.head.ref} - done`, PullRequestGitHelper.ID);
+
+		// Create local branch from the remote tracking branch
+		const trackedBranchName = `refs/remotes/${remoteName}/${pullRequest.head.ref}`;
+		const trackedBranch = await repository.getBranch(trackedBranchName);
+		progress.report({ message: vscode.l10n.t('Creating branch {0}', localBranchName) });
+		await repository.createBranch(localBranchName, false, trackedBranch.commit);
+
+		progress.report({ message: vscode.l10n.t('Checking out {0}', localBranchName) });
 		await repository.checkout(localBranchName);
 		// set remote tracking branch for the local branch
-		await repository.setBranchUpstream(localBranchName, `refs/remotes/${remoteName}/${pullRequest.head.ref}`);
+		await repository.setBranchUpstream(localBranchName, trackedBranchName);
 		await PullRequestGitHelper.associateBranchWithPullRequest(repository, pullRequest, localBranchName);
 	}
 

src/test/github/pullRequestGitHelper.test.ts

@@ -157,8 +157,10 @@ describe('PullRequestGitHelper', function () {
 				gitHubRepository,
 			);
 
-			repository.expectFetch('you', 'my-branch:pr/me/100');
-			repository.expectPull(true);
+			// Setup: Create remote tracking branch that will be created by fetch
+			await repository.createBranch('refs/remotes/you/my-branch', false, 'remote-commit-hash');
+
+			repository.expectFetch('you', 'my-branch');
 
 			const pullRequest = new PullRequestModel(credentialStore, telemetry, gitHubRepository, remote, prItem);
 
@@ -179,7 +181,7 @@ describe('PullRequestGitHelper', function () {
 			assert.deepStrictEqual(repository.state.HEAD, {
 				type: RefType.Head,
 				name: 'pr/me/100',
-				commit: undefined,
+				commit: 'remote-commit-hash',
 				upstream: {
 					remote: 'you',
 					name: 'my-branch',