Print full report for a single database diagnostics check

What does this MR do and why?

The gitlab:db:diagnostics rake task runs one or more database checks. When it runs many checks, it shows a menu so you can pick one to view. The per-check tasks, like gitlab:db:diagnostics:timeouts, only ever run one check. But they still showed the menu, with one entry and a quit option. This forced an extra step with no real choice. Now, when only one check runs, the task prints the summary and then the full details right away, with no menu. The menu still shows when more than one check runs. Output piped to a file or another program is not changed by this MR.

References

• #619095

Screenshots or screen recordings

Not applicable. This MR changes console output only.

How to set up and validate locally

1. Run bundle exec rake gitlab:db:diagnostics:timeouts in a terminal. You should see the summary, then the full details. No menu should appear.
2. Run bundle exec rake gitlab:db:diagnostics. You should still see the menu, since this task runs more than one check.

MR acceptance checklist

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