CVE-2014-9390 Vulnerability fix in libgit2

Updating libgit2 to 629417bd10d3308f1ee6fedd5bbb206a259337aa (latest commit in master) as it contains the fix for (CVE-2014-9390)[http://git-blame.blogspot.com/2014/12/git-1856-195-205-214-and-221-and.html] which fixes a vulnerability in git clients.