From GitHub’s announcement:
The vulnerability concerns Git and Git-compatible clients that access Git repositories in a case-insensitive or case-normalizing filesystem. An attacker can craft a malicious Git tree that will cause Git to ovewrite its own .git/config file when cloning or checking out a repository, leading to arbitrary command execution in the client machine. Git clients running on OS X (HFS+) or any version of Microsoft Windows (NTFS, FAT) are exploitable through this vulnerability. Linux clients are not affected if they run in a case-sensitive filesystem.
Better git to updating.