git: Add diff_base setting for showing changes since the default branch (#61501)

# Objective

Let git indicators — the editor gutter, file colors, and `git::Diff` —
show all changes on the current branch relative to its merge base with
the default branch, instead of only uncommitted changes.

Supersedes #60398; thanks to @samuelcolvin for the original
implementation and motivation.

Closes FR-135

## Solution

- New `git.diff_base` setting (`"head"` | `"default_branch"`), applied
live and toggleable per session from the editor controls menu ("Diff
Against Default Branch").
- Statuses come from a real merge-base-to-worktree tree diff (`git diff
--merge-base`), so local edits that revert branch changes correctly show
as unchanged.
- `GitStore` shares one `DiffBufferList` per repository with the Branch
Diff view; `repo_snapshots` and `project_path_git_status` keep returning
index/worktree truth, while display surfaces use separate `display_*`
APIs.
- `BufferDiff` now records what its base is (`DiffBaseKind`); hunks
whose base isn't HEAD are read-only in the gutter — stage/restore
buttons and keybindings are inert, so committed work can't be silently
rewritten.
- `git::Diff` follows the setting; new `git::DiffHead` always opens the
HEAD diff; `git::BranchDiff` is renamed `git::DiffBranch` (deprecated
alias kept).

Tradeoffs / known limitations:

- Hunk-level staging is unavailable while in `default_branch` mode
(whole-file staging via the git panel still works). Staging just the
uncommitted sub-ranges of a branch hunk is a follow-up.
- Remote hosts running an older server ignore the new
`GetTreeDiff.includes_worktree` proto field and degrade to
committed-changes-only branch diffs.
- Repositories with no resolvable default branch fall back to
HEAD-relative behavior; a failed first resolution retries on the next
branch-list change.

## Testing

- Real-git-repo tests for the merge-base-to-worktree diff's edge cases:
files recreated after index deletion, committed deletions recreated on
disk, and symlinks.
- GPUI tests for status semantics (a branch change reverted on disk
shows clean), `git::Diff` routing, live setting changes, and read-only
hunk enforcement (restore/stage leave buffer and index untouched).

## 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 adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

---

Release Notes:

- Git: Added a `git.diff_base` setting (`"head"` or `"default_branch"`)
that makes the editor gutter, file colors, and diff view show all
changes on the current branch since its merge base with the default
branch, instead of only uncommitted changes.

---------

Co-authored-by: Ben Kunkle <ben@zed.dev>