update to 2.21.3 (CVE-2020-11008)
    
    From the upstream release notes¹:
    
      With a crafted URL that contains a newline or empty host, or lacks
      a scheme, the credential helper machinery can be fooled into
      providing credential information that is not appropriate for the
      protocol in use and host being contacted.
    
      Unlike the vulnerability CVE-2020-5260 fixed in v2.17.4, the
      credentials are not for a host of the attacker's choosing; instead,
      they are for some unspecified host (based on how the configured
      credential helper handles an absent "host" parameter).
    
      The attack has been made impossible by refusing to work with
      under-specified credential patterns.
    
    ¹ https://www.kernel.org/pub/software/scm/git/docs/RelNotes/2.17.5.txt