LibGit2: fix toggle to use XOR instead of OR
The `toggle` function's docstring says it flips bits, but was using
`|=` (OR) which only sets bits. When a bit is already set, OR leaves
it as 1 instead of flipping it to 0. Use `⊻=` (XOR) for correct
toggle behavior.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
