Summary of changes

Security patch for LibGit2:

• 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.

Additionally:

• Stash should run much faster now.

Merged PRs into LibGit2

• Parallelize checkout_create_the_new for perf #4205
• stash: avoid recomputing tree when committing worktree
• Variadic macros
• Add sign capability to git_rebase_commit
• remote: remove unused block of code
• Adjust printf specifiers in examples code
• config: check if we are running in a sandboxed environment
• Fix example checkout to forbid rather than require --
• editorconfig: update to match our coding style
• Compare buffers in diff example
• Include ahead_behind in the test suite
• config: separate file and snapshot backends
• object: deprecate git_object__size for removal