Bump git version to 2.17.1
Git 2.17.1 is a security release, whose release notes can be found at https://github.com/git/git/blob/master/Documentation/RelNotes/2.17.1.txt
Git 2.17.1 is a security release, whose release notes can be found at https://github.com/git/git/blob/master/Documentation/RelNotes/2.17.1.txt