feat(diff-view): add Branch section for full-branch diff vs default

[SpielerNogard]

Description

Adds a Branch section to the Changes Panel that shows files differing between the project's default branch and the current task branch, with a Committed | All toggle. Committed mode shows defaultBranch...HEAD (three-dot via the existing mergeBaseRange); All mode shows git diff defaultBranch (working tree vs default, includes committed + staged + unstaged). Clicking a file opens a diff tab with the new diffGroup: 'branch'. The section is hidden when no default branch is configured.

A separate getMergeBase RPC resolves the merge-base SHA so Committed-mode diff tabs are pinned to the branch point. Without that pin, after a squash-merge the default and feature tips become content-equal and Monaco rendered an empty diff even though the file list (which uses three-dot semantics) still showed +N/-N. Branch tabs are read-only — no stage / discard / commit affordances — and the lifecycle store excludes them from the stale-tab sweep so they survive working-tree updates.

Related issues

None linked.

Testing

• pnpm run format — clean.
• pnpm run lint — clean (only pre-existing no-explicit-any warnings in tab-provider-registry.ts / task-tab-registry.tsx).
• pnpm run typecheck — clean.
• pnpm run test — 65/65 in the diff-view suite (incl. new tests in branch-diff-store.test.ts, diff-tab-manager.test.ts, and changes-view-store.test.ts); 1982/1982 in the wider non-browser suite.
• Manual: against a workspace snapshot — Branch section appears between Staged and PR, toggling Committed/All re-fetches files, clicking a file opens a (Branch) diff tab, default-branch task shows the On default branch empty state, branch tabs stay open during working-tree mutations, and merged-PR branches now render the expected red/green diff instead of an empty editor.

Screenshot/Recording (if applicable)

Before: When PR is open you can see the changed files

After the PR was merged, you no longer can access changed files

Now there is a branch view, where you can see everything changed on this branch

Checklist

• I kept this PR small and focused

• I ran a self-review before opening this PR

• I ran the relevant local checks or explained why not

• I updated docs when behavior or setup changed

• I added or updated tests when behavior changed, or explained why not

• I only added comments where the logic is not obvious

• I used Conventional Commits for commit messages and, when possible, the PR title

[greptile-apps]

Greptile Summary

This PR adds a Branch diff section to the Changes panel. The main changes are:

• New merge-base RPC support for worktree comparisons.
• A BranchDiffStore with Committed and All compare modes.
• Branch diff tabs, labels, model prefetching, and renderer support.
• Persisted branch compare mode and updated diff-view tests.

Confidence Score: 4/5

The Branch diff path can open or display comparisons against the wrong base ref.

• Remote default branches are not treated as the current local default branch.
• Stale merge-base requests can overwrite the active store state.
• Tabs opened before merge-base resolution can stay pinned to the live default branch.

branch-diff-store.ts and branch-section.tsx

Important Files Changed

Filename	Overview
apps/emdash-desktop/src/renderer/features/tasks/diff-view/stores/branch-diff-store.ts	Adds Branch diff state, compare modes, merge-base resolution, and file loading; remote/local default-branch equality and stale async merge-base writes need fixes.
apps/emdash-desktop/src/renderer/features/tasks/diff-view/changes-panel/branch-section.tsx	Adds the Branch section UI and opens Branch diff tabs; unresolved merge-base fallback can open tabs against the live default branch.
apps/emdash-desktop/src/main/core/git/worktree/controller.ts	Adds the getMergeBase RPC handler and returns the expected { sha } payload.
packages/core/src/git/git-worktree.ts	Adds core merge-base support through git merge-base.
apps/emdash-desktop/src/renderer/features/tasks/diff-view/main-panel/diff-file-renderer.tsx	Extends diff model registration and URI selection for Branch tabs.

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
  participant UI as BranchSection
  participant Store as BranchDiffStore
  participant RPC as gitWorktree RPC
  participant Git as Git service
  Store->>RPC: getMergeBase(defaultBranch, currentBranch)
  RPC->>Git: git merge-base base head
  Git-->>RPC: sha or null
  RPC-->>Store: merge-base result
  Store->>RPC: getChangedFiles(mergeBase...HEAD or mergeBase)
  RPC-->>Store: branch files
  UI->>Store: read files and mergeBaseRef
  UI->>UI: open branch diff tab

Loading

%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
sequenceDiagram
  participant UI as BranchSection
  participant Store as BranchDiffStore
  participant RPC as gitWorktree RPC
  participant Git as Git service
  Store->>RPC: getMergeBase(defaultBranch, currentBranch)
  RPC->>Git: git merge-base base head
  Git-->>RPC: sha or null
  RPC-->>Store: merge-base result
  Store->>RPC: getChangedFiles(mergeBase...HEAD or mergeBase)
  RPC-->>Store: branch files
  UI->>Store: read files and mergeBaseRef
  UI->>UI: open branch diff tab

Loading

Prompt To Fix All With AI

Fix the following 3 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 3
apps/emdash-desktop/src/renderer/features/tasks/diff-view/stores/branch-diff-store.ts:106
**Remote Default Branch Mismatch**

When the configured default branch resolves to `origin/main`, `defaultBranchRef` is a remote branch but `currentBranchRef` is always a local ref for `main`. `refsEqual` returns false for that type mismatch, so a worktree on the default branch skips the `on-default-branch` state and loads a Branch comparison against itself or its remote instead of showing the intended default-branch empty state.

### Issue 2 of 3
apps/emdash-desktop/src/renderer/features/tasks/diff-view/stores/branch-diff-store.ts:158-160
**Stale Merge Base Wins**

Disposing the old `Resource` does not cancel its in-flight `getMergeBase` call, but the callback still writes `_mergeBaseSha` on the shared store. If a default-branch, head, or compare-mode change starts a newer load and the older request finishes later, the file list can come from the new base while `mergeBaseRef` points at the old base, so clicking a Branch file opens a diff against the wrong branch point.

### Issue 3 of 3
apps/emdash-desktop/src/renderer/features/tasks/diff-view/changes-panel/branch-section.tsx:38
**Unresolved Base Opens Live Default**

Before `getMergeBase` resolves, this falls back to `defaultBranchRef`, and `openDiff` stores that live branch ref in the tab. A user who clicks a Branch file during that load window gets a tab pinned to the default branch tip instead of the branch point, and the tab keeps that wrong `originalRef` even after the store later resolves the merge base.

_{Reviews (1): Last reviewed commit: "fix(diff-view): pin branch diff to merge..." | Re-trigger Greptile}