libgit2 v0.28.3

This is a security release fixing the following issues:

• A carefully constructed commit object with a very large number
  of parents may lead to potential out-of-bounds writes or
  potential denial of service.

• The ProgramData configuration file is always read for compatibility
  with Git for Windows and Portable Git installations. The ProgramData
  location is not necessarily writable only by administrators, so we
  now ensure that the configuration file is owned by the administrator
  or the current user.