perf(git): parallelize git segment shell-outs to reduce prompt latency

[mroth]

Prerequisites

• I have read and understood the contributing guide.
• The commit message follows the conventional commits guidelines.
• Tests for the changes have been added (for bug fixes / features).
• Docs have been added/updated (for bug fixes / features).

Description

Restructure Enabled() to run independent git commands concurrently instead of sequentially, reducing the critical path from 5-7 serial git invocations down to 2 rounds.

Problem

With all features enabled (fetch_status, fetch_user, fetch_push_status, fetch_upstream_icon), the git segment executes up to 7 sequential git shell-outs:

1. setUser() → git config user.name, git config user.email
2. setStatus() → git status --porcelain=2
3. setHEADStatus() → git rev-parse, git describe (conditional)
4. setBranchStatus() → (no git calls)
5. setPushStatus() → git rev-list ahead, git rev-list behind
6. getUpstreamIcon() → git remote get-url (fallback only)

Changes

• Phase 1: setUser() runs concurrently alongside setStatus() via wg.Go
• Phase 2: After setStatus() completes, setHEADStatus(), setPushStatus(), and getUpstreamIcon() fan out in parallel
• setPushStatus() internally parallelizes its two rev-list calls
• setUser() consolidated from 2 git config calls to 1 via --get-regexp

Each goroutine writes to distinct fields on the Git struct — no synchronization beyond WaitGroup is needed. All goroutines are joined before Enabled() returns.

Benchmark results

Measured on Apple M4 Pro / 2TB SSD (median git segment duration, all features enabled):

Scenario	Before	After	Reduction
Small repo (native git)	34ms	19ms	44%
Small repo (50ms synthetic latency)	394ms	156ms	60%
Linux kernel (93k files)	320ms	302ms	6%
Kubernetes (25k files)	303ms	286ms	6%

The improvement is largest when per-call overhead is significant relative to git status time (small/medium repos, slow filesystems). On very large repos where git status dominates, the absolute savings are ~17-20ms.

Testing

• All existing tests pass (go test "./...")
• Race detector clean (go test -race)
• golangci-lint clean

Generated with assistance from Claude Code (claude-opus-4-6), but all code manually reviewed.

[Copilot]

Pull request overview

This PR reduces git segment prompt latency by restructuring Git.Enabled() to execute independent git shell-outs concurrently instead of serially, and by consolidating user config lookups into a single git invocation.

Changes:

• Parallelize setUser() with status collection and fan out additional git-dependent work after setStatus().
• Update setUser() to use git config --get-regexp instead of two separate git config calls.
• Parallelize the ahead/behind rev-list --count calls inside setPushStatus().

[JanDeDobbeleer]

Love this, great addition!