[2.7] bpo-29656: Handle PR branches in 'make patchcheck' (#302) (#628)

Additional changes needed to backport:

- dropped legacy SVN support from patchcheck
- use subprocess.PIPE to silence expected error output
- don't try to use subprocess.Popen as a context manager
- don't try to pass a keyword argument to str.split()

(cherry picked from commit 482f7a274fa52b7ba34ff308cd9acdcac9f41ba5)
1 file changed