Use Pull Request description as Squashed and Merged commit messages.

Fixes isaacs/github#1025

Chrome extension

Firefox add-on

When merging a PR with either the "Squash and merge" or "Create a merge commit" buttons:

1. The commit title and message will be copied from the Pull Request.
2. Any leading HTML comments will be removed.
3. Co-authors will be de-duplicated and appended.

The final commit on the base branch will look something like:

PR Title (#1234)

PR Body

Co-authored-by: ... (if multiple authors)

Click on the icon and select Enable Squashed Merge Message on this domain to enable the extension on your custom GitHub Enterprise domain.

In order for this extension to work with Refined GitHub you need to disable the clear-pr-merge-commit-message feature.

To disable in Chrome

1. Go to chrome://extensions
2. Open Details for Refined GitHub
3. Open Extension options
4. Search for and disable clear-pr-merge-commit-message

To disable in Firefox

1. Go to about:addons
2. Open Refined Github
3. Open Preferences
4. Search for and disable clear-pr-merge-commit-message

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