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>

crates/buffer_diff/src/buffer_diff.rs

@@ -25,6 +25,23 @@ pub struct BufferDiff {
     diff_snapshot: Option<BufferDiffSnapshot>,
     secondary_diff: Option<Entity<BufferDiff>>,
     buffer_snapshot: text::BufferSnapshot,
+    base_kind: DiffBaseKind,
+}
+
+/// Where this diff's base text came from. Only diffs whose base is HEAD
+/// support staging and restoring hunks; a diff against any other base (e.g.
+/// the merge base with another branch) would rewrite committed work.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum DiffBaseKind {
+    /// The buffer's committed (HEAD) content.
+    Head,
+    /// The buffer's index (staged) content.
+    Index,
+    /// An arbitrary blob, such as the merge base with another branch.
+    Oid,
+    /// Arbitrary caller-provided text, such as an agent's original text,
+    /// the clipboard, or another file.
+    Custom,
 }
 
 #[derive(Clone)]
@@ -1553,6 +1570,7 @@ impl BufferDiff {
         buffer: &text::BufferSnapshot,
         language: Option<Arc<Language>>,
         language_registry: Option<Arc<LanguageRegistry>>,
+        base_kind: DiffBaseKind,
         cx: &mut App,
     ) -> Self {
         let base_text = cx.new(|cx| {
@@ -1571,12 +1589,14 @@ impl BufferDiff {
             diff_snapshot: None,
             buffer_snapshot: buffer.clone(),
             secondary_diff: None,
+            base_kind,
         }
     }
 
     pub fn new_with_base_text_buffer(
         buffer: &text::BufferSnapshot,
         base_text_buffer: Entity<language::Buffer>,
+        base_kind: DiffBaseKind,
         _cx: &mut App,
     ) -> Self {
         BufferDiff {
@@ -1585,6 +1605,7 @@ impl BufferDiff {
             diff_snapshot: None,
             buffer_snapshot: buffer.clone(),
             secondary_diff: None,
+            base_kind,
         }
     }
 
@@ -1592,6 +1613,7 @@ impl BufferDiff {
         buffer: &text::BufferSnapshot,
         language: Option<Arc<Language>>,
         language_registry: Option<Arc<LanguageRegistry>>,
+        base_kind: DiffBaseKind,
         cx: &mut Context<Self>,
     ) -> Self {
         let base_text = buffer.text();
@@ -1622,6 +1644,7 @@ impl BufferDiff {
             diff_snapshot: Some(diff_snapshot),
             buffer_snapshot: buffer.clone(),
             secondary_diff: None,
+            base_kind,
         }
     }
 
@@ -1631,7 +1654,7 @@ impl BufferDiff {
         buffer: &text::BufferSnapshot,
         cx: &mut Context<Self>,
     ) -> Self {
-        let mut this = BufferDiff::new(buffer, None, None, cx);
+        let mut this = BufferDiff::new(buffer, None, None, DiffBaseKind::Head, cx);
         let mut base_text = base_text.to_owned();
         text::LineEnding::normalize(&mut base_text);
         let base_text_buffer = cx.new(|cx| {
@@ -1655,6 +1678,16 @@ impl BufferDiff {
         self.secondary_diff = Some(diff);
     }
 
+    pub fn base_kind(&self) -> DiffBaseKind {
+        self.base_kind
+    }
+
+    /// Whether hunks in this diff can be staged or restored: true only when
+    /// the diff's base is HEAD.
+    pub fn is_stageable(&self) -> bool {
+        self.base_kind == DiffBaseKind::Head
+    }
+
     pub fn secondary_diff(&self) -> Option<Entity<BufferDiff>> {
         self.secondary_diff.clone()
     }
@@ -2449,7 +2482,8 @@ mod tests {
             ],
         );
 
-        diff = cx.update(|cx| BufferDiff::new(&buffer, None, None, cx).snapshot(cx));
+        diff = cx
+            .update(|cx| BufferDiff::new(&buffer, None, None, DiffBaseKind::Head, cx).snapshot(cx));
         assert_hunks::<&str, _>(
             diff.hunks_intersecting_range(
                 Anchor::min_max_range_for_buffer(buffer.remote_id()),
@@ -3154,7 +3188,8 @@ mod tests {
 
         let mut buffer = Buffer::new(ReplicaId::LOCAL, BufferId::new(1).unwrap(), buffer_text_1);
 
-        let empty_diff = cx.update(|cx| BufferDiff::new(&buffer, None, None, cx).snapshot(cx));
+        let empty_diff = cx
+            .update(|cx| BufferDiff::new(&buffer, None, None, DiffBaseKind::Head, cx).snapshot(cx));
         let diff_1 = BufferDiffSnapshot::new_sync(&buffer, base_text.clone(), cx);
         let DiffChanged {
             changed_range,
@@ -4312,7 +4347,8 @@ mod tests {
         );
         let buffer_snapshot = buffer.snapshot();
 
-        let diff = cx.new(|cx| BufferDiff::new(&buffer_snapshot, None, None, cx));
+        let diff =
+            cx.new(|cx| BufferDiff::new(&buffer_snapshot, None, None, DiffBaseKind::Head, cx));
         diff.update(cx, |diff, cx| {
             diff.set_base_text(Some(Arc::from(base_text_crlf)), buffer_snapshot.clone(), cx)
         })