Describe the feature or problem you’d like to solve

When performing a squash and merge on a pull request through the web interface, you can edit the commit message and extended description. By default, this is prepopulated with a list of all of the commit messages included in the pull request. I often find myself deleting some of the lines from this section because the additional commits are usually related to feedback brought up during the code review, and would be irrelevant or confusing when viewing in the context of the final commit.

When merging a PR through the CLI, the same commit message is generated as the web interface, but there is no option to edit the commit message.

Proposed solution

Open the user's default terminal editor to allow the PR maintainer to enter the commit message. By default, prepopulate the editor with the default generated message so the user can easily just save and exit the editor to continue using the current behaviour.

SPEC: #1023 (comment)