checkout: add `--relink` to tab completion scripts

[jorgeorpinel]

Per #2492

And are there any other relatively newly added/changed/removed command options we should add to the tab completion files? Or commands, for that matter... Moved to #2985

[jorgeorpinel]

And are there any other relatively newly added/changed/removed command options

e.g. gc: remove --all-commits options from usage output #2681

[jorgeorpinel]

Another e.g.: dvc run --remove-outs is now deprecated.

[shcheklein]

@Suor could you please take care of the --relink, please? It was part of the PR you created, right?

[jorgeorpinel]

I'll move everything else to #2985 so this issue is just about --relink now.