Set upstream tracking when creating worktree for existing local branch

When a local branch already exists and a matching remote branch is
found, the worktree was created without configuring upstream tracking.
This meant git pull/push required manual upstream setup.

Author: ZeroErrors

src/git-wt/Commands.cs

@@ -41,6 +41,9 @@ public static int Create(string branchName)
             {
                 Console.WriteLine($"Using existing branch '{branchName}'...");
                 exitCode = Git.RunLive(bareRepoPath, "worktree", "add", worktreePath, branchName);
+
+                if (exitCode == 0 && remoteBranchExists)
+                    Git.Run(bareRepoPath, "branch", $"--set-upstream-to=origin/{branchName}", branchName);
             }
             else if (remoteBranchExists)
             {