dev-cmd/bump: increase test coverage

[nandahkrishna]

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This is a follow-up PR to increase brew bump's test coverage after the refactoring PR (#10285).

Summary of changes:

• Combined repetitive logic into a single method, retrieve_and_display_info.
• Added a test for HEAD-only formulae.

In hindsight, I shouldn't have removed the do not merge label from the refactoring PR, I didn't notice the decrease in coverage until Sam mentioned it. Sorry! 😅

CC @samford

[BrewTestBot]

Review period will end on 2021-01-26 at 11:49:27 UTC.

[MikeMcQuaid]

In hindsight, I shouldn't have removed the do not merge label from the refactoring PR, I didn't notice the decrease in coverage until Sam mentioned it. Sorry! 😅

Don't worry, I didn't see it as a blocking issue.

[BrewTestBot]

Review period ended.