dev-cmd/bump: reduce unnecessary output

[EricFromCanada]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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When using brew bump with --no-pull-requests, omit the open & closed pull requests lines which will always be "none" and may therefore be inaccurate.

Also added a method to always skip both head-only and disabled formulae. This fixes an issue that would occur with the command's non-argument form if a matched formula was head-only.

[nandahkrishna]

Makes sense to me, thanks!

[MikeMcQuaid]

Good idea, thanks @EricFromCanada!