Show worktree name next to branch in branches list

[ruudk]

PR Description

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

When a branch is checked out by another worktree, show the worktree name in the label, e.g. "(worktree cosmos2)" instead of just "(worktree)", so you can immediately see which worktree holds it.