xfail test: double-push with conflict *and* non-English locale
Set HGPLAIN env variable at start of gitifyhg, closes #26

In turn, do not set it in the test suite, as this was covering up various
potential test failures caused by the locale not being set to English.

This fixes the xfail test added in the previous commit.
Merge pull request #42 from fingolfin/hg-plain

Set HGPLAIN env variable at start of gitifyhg, closes #26