On an average weekday, more than 70,000 comments are added to discussions on GitHub.
Today we’re introducing a small enhancement to the way you can reply to discussion
threads to make it even easier. Pressing the r key on your keyboard is now a
shortcut to reply to the current issue or pull request with a comment. That’s not all,
however. Any text you select within the discussion thread
before pressing r will be added to your comment and formatted as a blockquote
using GitHub Flavored Markdown.

GitHub quick reply quoting

For more details on the keyboard shortcuts that we support, hit the ? key on any page.