gitconfig - point `core.excludesfile` to gitignore_global

So we can actually make use of our `gitignore_global` file in this
repository as the global `.gitignore` file.

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
gitignore_global - Ignore some OCaml build products

Side effects of working on CS4212 assignment

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>