Mercurial Backend: HGPLAIN env var

[lukaszb]

{{{
def plain(self):
'''is plain mode active?
Plain mode means that all configuration variables which affect the
behavior and output of Mercurial should be ignored. Additionally, the
output should be stable, reproducible and suitable for use in scripts or
applications.
The only way to trigger plain mode is by setting the `HGPLAIN'
environment variable.
"""
}}}

this is mercurial.ui.ui method and a docstring. We should set that 'HGPLAIN' environment variable probably at the mercurial backend's module level as it would stabilize the output - could be helpful with workdirs.

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Reference: https://bitbucket.org/marcinkuzminski/vcs/issues/28/