hghave: add "chg" flag to skip tests that can't be compatible with chg Several tests fail with chg for several reasons such as loaded chgserver extension, running uisetup() per server instead of per runcommand, etc. Since these tests can't/shouldn't be changed to be chg friendly, we need a flag to skip them. This patch explicitly drops CHGHG environment if chg isn't involved. This way, hghave can just check if CHGHG exists.