Spawn: Fix O(N²) string parsing

[MaxFangX]

Fixes a performance issue in Spawn:parse_result() that was creating excessive string allocations when parsing git command output.

The previous implementation used character-by-character iteration:

for i = 1, #output do
    local char = output:sub(i, i)  -- Creates a new 1-char string
    -- ... accumulate in table ...
end

For large git output (e.g., git ls-files returning 20KB), this created ~20,000 individual string objects. While the algorithmic complexity is O(N), the constant factor from N allocations + table operations made this unnecessarily slow.

This PR replaces it with efficient newline-based splitting using string.find(): local newline_pos = output:find('\n', start, true) callback(output:sub(start, newline_pos - 1))

This creates only one substring per line (not per character), dramatically reducing allocations.

Testing with a 932-file repository showed git ls-files parsing improved from 796ms → 594ms (25% improvement), and overall stage operations improved from 1605ms → 1201ms. Existing functionality unchanged - all git command parsing works as before.