fix(facts.git): GitLocalCommit fact failing on nonlocal ref

[yagarea]

Fixes: #1880

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Its docstring explicitly promises: "returns None when the repository does not exist, the ref is unknown, or the command fails." And the process() method is written for that — it validates the output against a SHA regex and returns None otherwise.

But the shell command doesn't hold up its end. When the directory exists and the ref is unknown, git rev-parse exits with code 128. The 2>/dev/null hides the error text but does nothing about the exit code, so the whole fact command exits non-zero. pyinfra treats a failed fact command as fatal — Error: could not load fact — kicks the host out, and you get No hosts remaining!.

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As per AI policy I disclose that test were created with assistance of LLM. I have read and tested them manually so its human verified.

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• Pull request is based on the default branch (3.x at this time)
• Pull request includes tests for any new/updated operations/facts
• Pull request includes documentation for any new/updated operations/facts
• Tests pass (see scripts/dev-test.sh)
• Type checking & code style passes (see scripts/dev-lint.sh)
• Pull request title follows the
  conventional commits format