gh pull-request doesn't suggest PR title/message

When there's only one commit on the branch, `hub pull-request` uses that commit message as the default for the PR message. `gh pull-request` didn't do that for me:

```
# Requesting a pull to phinze:master from phinze:pckeyboardhack-10.4.0
#
# Write a message for this pull request. The first block
# of the text is the title and the rest is description.
#
# Changes:
#
# 691d920 (Peter Jaros, 18 minutes ago)
#    Upgrade pckeyboardhack to 10.4.0.
```