Function resolve_diff_context_lines 

pub fn resolve_diff_context_lines(
    cfg: &ConfigSet,
) -> Result<Option<usize>, String>

Read diff.context from a loaded ConfigSet with Git-compatible validation.

Returns Ok(None) when the key is unset. When set, the value must be a non-negative integer acceptable to Git’s diff machinery (same rules as git diff / git log -p).