chore: remove redundant nonisolated on methods inside nonisolated GitFetcher enum

[batonogov]

Summary

After PR #657 (Swift 6 migration), enum GitFetcher is marked nonisolated, but individual static methods inside it still have their own nonisolated annotations. These are redundant — members of a nonisolated enum inherit the isolation.

Motivation

Code cleanliness. Redundant annotations add noise and may confuse future contributors.

Implementation ideas

• Remove nonisolated from individual methods inside GitFetcher in GitStatusProvider.swift
• Same for GitLineDiff static methods (Sendable struct, nonisolated is implicit)