[Buildbot] #3180: Fix call to subprocess.Popen in git_buildbot.py

[Buildbot trac]

#3180: Fix call to subprocess.Popen in git_buildbot.py
-------------------+-----------------------
Reporter:  verm    |      Owner:
    Type:  defect  |     Status:  new
Priority:  major   |  Milestone:  undecided
 Version:  master  |   Keywords:  git
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 [3add6d54] changed `git_buildbot.py` to use subprocess.Popen.  On Windows
 I believe it is always `shell=True` which is why passing the arguments as
 a string works.  On Unix it must be a list (as far as I know).  I do not
 know if it will cause any issues on Windows.

 skelly on IRC mentioned this may have been meant as `os.popen` instead of
 `subprocess.Popen`

 See
 [https://github.com/buildbot/buildbot/blob/master/master/contrib/git_buildbot.py#L242
 git_buildbot.py:242].

 This is on FreeBSD 10.1 under Python 2.7.

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Ticket URL: <http://trac.buildbot.net/ticket/3180>
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