SPARK-1073 Keep GitHub pull request title as commit summary

The first line of a git commit message is the line that's used with many git
tools as the most concise textual description of that message.  The most
common use that I see is in the short log, which is a one line per commit
log of recent commits.

This commit moves the line

  Merge pull request #%s from %s.

Lower into the message to reserve the first line of the resulting commit for
the much more important pull request title.

http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html