When BAT_CONFIG_DIR is set to the system config directory (e.g. /etc/bat),
both system_config_file() and config_file() resolve to /etc/bat/config.
The config file was read twice, causing clap errors for non-repeatable flags
like --italic-text.
Skip the user config read when it resolves to the same file as the system
config, using canonicalization to also handle symlinks.
Closes#3589