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

The first commit adds a new xfail test to demonstrate issue #26 explicitly.

The second commit adds a fix for the issue by setting the HGPLAIN env var at the start of gitifyhg. At the same time, it changes the test harness to *not* set it as part of the test fixture, as this was covering up potential test failures. And it should be obsolete now, too, due to us setting it in gitifyhg directly.