CINCINNATI — The Giants had struggled to scrape together runs for two games against one of the worst teams in the majors and appeared to be headed down an even worse path in Sunday’s series finale. They were held hitless into the seventh inning and scoreless until the eighth but were kept in the game by another strong pitching performance from starter Alex Cobb. And with a six-run rally off Reds relievers in the eighth inning — highlighted by Evan Longoria’s go-ahead three-run home run — they were able to stave off a sweep with a 6-4 win at Great American Ballpark. Longoria’s three-run shot in…