Rebecca Davis editorKeira Knightley was “incredibly pregnant” the first time she read writer-director Camille Griffin’s script for the comedic holiday horror-drama “Silent Night,” and recalls finding the unusual tale of a Christmas Eve gathering the night of a coming apocalypse “absolutely, hysterically funny.”She read it again months later — sleep deprived and with her six-week-old daughter in tow — and still found it hilarious.But by the time production started, her daughter was five months