Make hook context input reads thread-safe

[Felix-LeeSM]

Summary

This fixes a small race in HookContext::Base#input_string.

Hook contexts currently cache stdin lazily with @input_string ||= @input.read. Since hooks are parallelized by default, two hooks sharing the same context can race during that first read. For hook types that receive real data on stdin, such as pre-push, one hook can consume the stream while another reads EOF and caches an empty string.

If that happens, helpers like pushed_refs can silently see no refs even though Git did provide them.

Changes

• Add a mutex around the first input_string read so the stream is consumed once.
• Keep the cached string shared by all callers after the initial read.
• Add a spec that forces a concurrent read attempt without relying on flaky timing.

This follows up on #881.

Verification

Focused checks pass locally:

bundle exec rspec spec/overcommit/hook_context/base_spec.rb
bundle exec rubocop lib/overcommit/hook_context/base.rb spec/overcommit/hook_context/base_spec.rb
bundle exec overcommit --run

I also ran the full spec suite. It still has a few unrelated failures around git alias amendment detection on my machine, but this change's focused spec passes.