## Context

I have a project with a nested git repo inside a parent repo (separate
`.git` directory, not a submodule). The title bar shows the parent's
branch instead of the nested repo's own branch.

The issue is in `get_repository_for_worktree()` — it iterates
`git_store.repositories()` (an `FxHashMap`) and returns the first
`starts_with` match. Both the parent and nested repo match, and
whichever one FxHashMap iterates first wins. There's no reason it should
be one or the other.

The fix already exists elsewhere in the codebase —
`GitStore::repository_and_path_for_project_path()` at
`git_store.rs:1826` uses `.max_by_key()` to pick the most specific
(longest path) match. This PR applies the same approach to three
functions that have the same problem:

- `TitleBar::get_repository_for_worktree()` — branch display in title
bar
- `resolve_active_repository()` in `git_ui` — repository selection for
the git panel
- `get_branch_for_worktree()` in `recent_projects` — branch display in
the project switcher

Two other locations use a similar `starts_with` pattern
(`effective_active_worktree()` in `title_bar.rs` and worktree selection
in `recent_projects.rs`) but those iterate worktrees against a single
known repo, not repos against a worktree — so first-match and
longest-match give the same result. Left those unchanged.

Closes #7566

## How to Review

All three changes are the same transformation: first-match loop (or
`.find()`) → `.filter().max_by_key()` on path length. The reference is
at `crates/project/src/git_store.rs:1826`.

The primary fix is `get_repository_for_worktree()` in `title_bar.rs`.
The other two are the same pattern.

One difference from the reference: I used `.as_os_str().len()` instead
of `.clone()` for the `max_by_key` key — avoids cloning an `Arc<Path>`
per comparison. Among prefix-related paths (which is all that passes the
filter), the longer path is always the more specific match, so length
comparison is equivalent.

`title_bar` has no existing test infrastructure. Happy to add a test if
you'd like — the setup would follow the pattern in
`test_git_traversal_with_nested_repos`
(`crates/project/tests/integration/git_store.rs`).

```
cargo test -p title_bar -p git_ui -p recent_projects
./script/clippy
```

## Self-Review Checklist

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Fixed branch picker showing parent repository's branch instead of the
nested repository's branch when working in submodules or nested git
repos.

---------

Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>
Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>