BERLIN (Reuters) -Puma Chief Executive Bjorn Gulden is leaving the German sportswear maker at the end of the year to take the helm at rival Adidas, according to an industry source. Puma announced on Friday that it had appointed Chief Commercial Officer Arne Freundt as the new CEO from Jan. 1 after Gulden decided to let his contract expire at the end of the year, but did not say what Gulden’s plans were. Freundt, who has worked for Puma for over 10 years and was promoted to CCO last year, will receive a four-year contract. Adidas confirmed on Friday that it is in talks with Gulden about possibl…