CheckoutPR: Add support for updating main

[koppor]

Wished at JabRef/blog.jabref.org#115 (comment)

Works:

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• [/] Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• [/] Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (if change is visible to the user)
• [/] Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[trag-bot]

Avoid using exclamation marks at the end of a sentence. They are more for screaming. Use a dot to end the sentence.

[trag-bot]

Avoid using exclamation marks at the end of a sentence. They are more for screaming. Use a dot to end the sentence.

[subhramit]

tiny questions/comments on uniformity
update - found a comment in the file, can ignore.

[subhramit]

I think usage of the command line pgm calls is intended, if we want the verbose git outputs on CLI, so looks good to me.