set HGPLAIN environment variable when invoking hg process

[vladak]

As hg(1) man page says:

       HGPLAIN
              If not set, Mercurial's output could be influenced by configura-
              tion settings that impact its encoding, verbose mode,  localiza-
              tion, etc.

              It  is highly recommended for machines to set this variable when
              invoking hg processes.

[vladak]

Given that setting env var in Java is not a simple task, we might well defer this till Java 9 which looks like it will be finally systems friendly.