Support for git global options

[chitoku-k]

Command name

git

Completion context

git -C /path/to/cwd checkout **<TAB>
git -c foo=bar checkout **<TAB>
git --config-env=foo=BAR_VALUE checkout **<TAB>
git --exec-path=/path/to/git checkout **<TAB>
git --git-dir=/path/to/.git checkout **<TAB>
git --work-tree=/path/to/work-tree checkout **<TAB>
git --namespace=namespace **<TAB>
git --attr-source=tree-ish **<TAB>

What happens now?

Files are completed.

What do you expect to complete?

Proper completion is provided.

Currently, the second argument is always treated as a subcommand, which in this case always breaks completion. The best is to implement every global option but it should at least ignore them and correctly extract a subcommand.

[hexium310]

Can you commonize the treatment of global options in the completion of kubectl? This can easily implement completions accounting for global options.

[chitoku-k]

I've created #238. Does it suffice?