Describe the feature or problem you’d like to solve

When opening a pull request and editing the body of the request, I wish the commit messages were available in the editor.

This is a feature I miss from hub. When you'd run hub pull-request create and the editor was opened, there was a section:

# ------------------------ >8 ------------------------
# Do not modify or remove the line above.
# Everything below it will be ignored.

And below it would be all the commit messages that make up this pull request.

While --fill-first is nice, I often want to incorporate other commit messages into my pull request description. The hub way of doing this made that very easy to do. As it is today, I find myself running git log in a terminal window and cut-and-pasting today.

Proposed solution

An identical implementation to hub's would be fine with me. There may be other ways to achieve the same outcome.