git diff uses bare branch name instead of origin/ prefix, fails when no local branch exists

Bug Description

Claude Code's internal getDefaultBranch function resolves the repo's default branch name (e.g., develop, main) by reading refs/remotes/origin/HEAD, but returns just the branch name without the origin/ prefix. This value is then used in a git diff --name-only HEAD <branch> command, which fails if no local branch with that name exists.

Steps to Reproduce

1. Have a repo where origin/HEAD points to a branch (e.g., origin/develop) that does not exist as a local branch
2. Run git push (or any action that triggers Claude Code's pre-push git diff)
3. Observe the error:

> git diff --name-only HEAD develop
fatal: ambiguous argument 'develop': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

Expected Behavior

The command should use origin/develop (or origin/<branch>) instead of the bare branch name, since the branch was resolved from a remote ref and may not exist locally.

Root Cause

In the Claude Code binary, the getDefaultBranch function reads refs/remotes/origin/HEAD and strips the refs/remotes/origin/ prefix, returning just the branch name (e.g., "develop"). The caller then passes this directly to git diff --name-only HEAD develop without prepending origin/.

Relevant logic (deминified):

async function getDefaultBranch() {
  let gitDir = await getGitDir();
  if (!gitDir) return "main";
  let commonDir = await getCommonDir(gitDir) ?? gitDir;
  // Reads refs/remotes/origin/HEAD -> returns bare name like "develop"
  let branch = await readSymRef(commonDir, "refs/remotes/origin/HEAD", "refs/remotes/origin/");
  if (branch) return branch;
  for (let name of ["main", "master"])
    if (await refExists(commonDir, `refs/remotes/origin/${name}`)) return name;
  return "main";
}

Workaround

Either:

• Create a local tracking branch: git fetch origin <branch>:<branch> (only works if the branch actually exists on the remote)
• Fix origin/HEAD if it's stale: git remote set-head origin <actual-default-branch>

Environment

• Claude Code version: 2.1.96 (VS Code extension, anthropic.claude-code-2.1.96-darwin-arm64)
• OS: macOS (Darwin, arm64)