Potential fix for code scanning alert no. 1: Unsafe shell command constructed from library input

[n-rodriguez]

Potential fix for https://github.com/redmine-git-hosting/grack/security/code-scanning/1

To fix this without changing intended functionality, validate/allowlist the dynamic config key input before it is passed into git execution.

Best approach in this file:

• In config_setting(service_name), normalize as today, then enforce service_name to be only known safe values used by this code (uploadpack, receivepack).
• In config(config_name), enforce a strict format for config keys accepted by this class (here, http.uploadpack / http.receivepack), and reject anything else with ArgumentError.
• Keep using array-based IO.popen as-is.

This addresses all alert variants because both taint sources (service_name, config_name) are constrained before reaching execute/command.

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