CRAN release v0.28.0

IMPROVEMENTS

• Updated to use libgit2 version 1.1.0 on Windows.

• Fix handling of a symbolic reference when checking out previous
  branch.

• Added a configure option '--without-libgit2' to ignore presence of a
  system libgit2 library and instead use the internal git2r libgit2
  library. Usage:
  R CMD INSTALL --configure-args='--without-libgit2' git2r_x.y.z.tar.gz

• Updated some tests to work with libgit2 version 1.1.0.