PHILADELPHIA, Sept 10 (Reuters) - Democratic Vice President Kamala Harris put Republican Donald Trump on the defensive at a combative presidential debate on Tuesday with a stream of attacks on abortion limits, his fitness for office and his myriad legal woes, as both candidates sought a campaign-altering moment in their closely fought election. In a boost to the Harris campaign, pop megastar Taylor Swift told her 283 million followers on Instagram in a post immediately...