fix(repo): sync a branch without switching to it

tea repo sync ran git switch on the synced branch, so syncing main while
on experiment left the user on main. A branch that is not checked out now
moves with update-ref, guarded by its old value; the checked-out branch
merges with --ff-only and needs a clean working tree; a missing branch is
created and set to track the remote. Diverged branches stay untouched.

Sync reports "✓ Synced main from origin/main (fast-forward a..b)" on a
terminal, "already up to date" when nothing changed, and the JSON result
gains the previous commit. Outside a clone, or without a remote for the
repository, the errors now name repo sync instead of checkout.

Signed-off-by: bircni <bircni@icloud.com>

internal/app/repos_test.go

@@ -0,0 +1,18 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package app
+
+import (
+	"bytes"
+	"testing"
+)
+
+// repoTTYDeps returns serverDeps for a 100-column terminal without color or links, as people read tea's output.
+func repoTTYDeps(t *testing.T, serverURL string) (Dependencies, *bytes.Buffer, *bytes.Buffer) {
+	t.Helper()
+	deps, out, diagnostics := serverDeps(t, serverURL)
+	deps.Getenv = serverEnv(serverURL, map[string]string{"TERM": "dumb"})
+	deps.Streams.OutTTY, deps.Streams.Width = true, 100
+	return deps, out, diagnostics
+}