fix(git): resolve the base branch from the repository instead of its name

Follow-up to #2629, which stopped the walkthrough from comparing against a
branch that does not exist. The same guessing, and the same near-misses in how
the answer was applied, were left elsewhere:

- The default branch travelled as `rootBranchHint`, whose documented meaning is
  "the branch the project root worktree is on". It gets its own option, because
  a parameter that means two things is one the next caller gets wrong.
- A candidate equal to the branch being compared is skipped. In a plain checkout
  the root hint *is* the current branch, so it won every time and produced a
  comparison with itself; the repository default now wins there.
- The Changes and pull-request surfaces read the default branch too. A pull
  request opened against a branch that does not exist is a worse failure than a
  walkthrough that will not generate.
- `hasResolvableBaseBranch` matched `origin/feature/main` for a base of `main`,
  passing the check and then failing the comparison it exists to prevent.
- `getRangeDiff` promoted only `origin/<base>`. A base carried by any other
  remote stayed a bare name, which git resolves against refs/heads and nowhere
  else, so it failed exactly as before.
- `getBranches` dropped every branch of a remote that did not answer, turning
  "we could not ask" into "these branches are gone" — offline, that silently
  removed comparisons that work fine against local remote-tracking refs.
- A remote with no `remote/HEAD` is asked once with `ls-remote --symref` rather
  than falling back to the guess this data exists to replace.

The `defaultBranches` contract was documented under the status response; it
belongs to the branches response, which now has a section of its own.

packages/web/server/lib/git/service.test.js

@@ -11,6 +11,7 @@ import {
   createWorktree,
   getWorktreeBootstrapStatus,
   getBranches,
+  getRangeDiff,
   getStatus,
   isGitRepository,
   populateWorktreeWithLockRecovery,
@@ -48,6 +49,28 @@ const runGit = (cwd, args) =>
     stdio: ['ignore', 'pipe', 'pipe'],
   });
 
+/**
+ * A repository on `next` whose only remote publishes `defaultBranch` and has it
+ * recorded as that remote's HEAD — the shape of every repository whose default
+ * branch is not one of the conventional names.
+ */
+const createRepositoryWithRemote = ({ remoteName = 'origin', defaultBranch = 'react' } = {}) => {
+  const remote = createTempDir();
+  const repository = createTempDir();
+  runGit(remote, ['init', '--bare', `--initial-branch=${defaultBranch}`]);
+  runGit(repository, ['init', '-b', 'next']);
+  runGit(repository, ['config', 'user.email', 'test@example.com']);
+  runGit(repository, ['config', 'user.name', 'Test']);
+  fs.writeFileSync(path.join(repository, 'README.md'), '# Test\n');
+  runGit(repository, ['add', 'README.md']);
+  runGit(repository, ['commit', '-m', 'init']);
+  runGit(repository, ['remote', 'add', remoteName, remote]);
+  runGit(repository, ['push', remoteName, `HEAD:${defaultBranch}`]);
+  runGit(repository, ['fetch', remoteName]);
+  runGit(repository, ['remote', 'set-head', remoteName, '--auto']);
+  return { remote, repository };
+};
+
 const canRunGit = () => {
   try {
     execFileSync('git', ['--version'], { stdio: 'ignore' });
@@ -992,22 +1015,47 @@ describe('hash validation', () => {
 
 describe.runIf(canRunGit())('getBranches', () => {
   it('returns a remote default branch whose name is not a conventional fallback', async () => {
-    const remote = createTempDir();
-    const repository = createTempDir();
-    runGit(remote, ['init', '--bare', '--initial-branch=react']);
-    runGit(repository, ['init', '-b', 'next']);
-    runGit(repository, ['config', 'user.email', 'test@example.com']);
-    runGit(repository, ['config', 'user.name', 'Test']);
-    fs.writeFileSync(path.join(repository, 'README.md'), '# Test\n');
-    runGit(repository, ['add', 'README.md']);
-    runGit(repository, ['commit', '-m', 'init']);
-    runGit(repository, ['remote', 'add', 'origin', remote]);
-    runGit(repository, ['push', 'origin', 'HEAD:react']);
-    runGit(repository, ['fetch', 'origin']);
-    runGit(repository, ['remote', 'set-head', 'origin', '--auto']);
+    const { repository } = createRepositoryWithRemote({ remoteName: 'origin', defaultBranch: 'react' });
 
     await expect(getBranches(repository)).resolves.toMatchObject({
       defaultBranches: { origin: 'react' },
     });
   });
+
+  it('asks the remote when no local remote/HEAD exists', async () => {
+    const { repository } = createRepositoryWithRemote({ remoteName: 'origin', defaultBranch: 'react' });
+    // A hand-added remote can end up without this ref; the branch it points at
+    // is still knowable, and guessing instead is the bug this data replaces.
+    runGit(repository, ['remote', 'set-head', 'origin', '--delete']);
+
+    await expect(getBranches(repository)).resolves.toMatchObject({
+      defaultBranches: { origin: 'react' },
+    });
+  });
+
+  it('keeps the branches of a remote that cannot be reached', async () => {
+    const { repository, remote } = createRepositoryWithRemote({ remoteName: 'origin', defaultBranch: 'react' });
+    fs.rmSync(remote, { recursive: true, force: true });
+
+    const branches = await getBranches(repository);
+
+    // "We could not ask" is not "the branch is gone": callers read this list to
+    // decide whether a base branch exists at all.
+    expect(branches.all).toContain('remotes/origin/react');
+  });
+});
+
+describe.runIf(canRunGit())('getRangeDiff', () => {
+  it('resolves a base that exists only on a remote other than origin', async () => {
+    const { repository } = createRepositoryWithRemote({ remoteName: 'upstream', defaultBranch: 'react' });
+    // Only refs/remotes/upstream/react carries the base — git cannot resolve the
+    // bare name, so an unqualified `react...next` fails with "ambiguous argument".
+    fs.writeFileSync(path.join(repository, 'feature.txt'), 'work\n');
+    runGit(repository, ['add', 'feature.txt']);
+    runGit(repository, ['commit', '-m', 'feature']);
+
+    const diff = await getRangeDiff(repository, { base: 'react', head: 'next' });
+
+    expect(diff).toContain('feature.txt');
+  });
 });