MiniMax Code CLI 0.3.1 added a status-line segment showing the git branch
for the current workspace. Hermes' status bar had no repo-awareness field.
Adds `git_branch` to display.status_bar.fields (opt-in only — the default
set never probes the filesystem). Reads .git/HEAD directly with a 5s
per-directory TTL cache (no subprocess per repaint); follows gitdir:
pointer files so worktrees/submodules resolve their private HEAD; a
detached HEAD renders the abbreviated commit.
Inspired by MiniMax Code CLI 0.3.1 changelog (agent.minimax.io/docs/changelog).
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