[Buildbot] #3474: Add Pull Request template on GitHub

[Buildbot trac]

#3474: Add Pull Request template on GitHub
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Reporter:  rutsky     |      Owner:
    Type:  undecided  |     Status:  new
Priority:  major      |  Milestone:  undecided
 Version:  master     |   Keywords:
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 Recently GitHub introduced [https://github.com/blog/2111-issue-and-pull-
 request-templates pull request templates]: we can add template for new
 pull requests.
 When user creates pull request, it's body will be filled with template
 contents.

 Currently body is filled with commit message if PR consists of single
 commit, and it's empty if PR is of multiple commits.

 Do we want to use this feature?

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Ticket URL: <http://trac.buildbot.net/ticket/3474>
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Buildbot: build/test automation