annotate required flags for bash_completions

[eparis]

some commands require that certain flags be set in order to function.
Annotate those flags so the bash completions will suggest to the user
that they be set.

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[fgrzadkowski]

lgtm

[fgrzadkowski]

I think that shippable failure is just a flake, but can you please take a look?

[eparis]

shippable is clean....

[googlebot]

CLAs look good, thanks!