0.28.4.1: Tests fail/segfault using libgit2 0.28.4 or 0.99.0 sources

On Arch Linux we're currently trying to rebuild dependants against the latest libgit2 release (0.99.0).

Rugged is giving trouble trying to use the 0.99.0 sources for testing:

  mv -v ../libgit2-${_libgit2_pkgver}/* vendor/libgit2
  export CI_BUILD=true
  export RUGGED_USE_SYSTEM_LIBRARIES=true
  export CMAKE_FLAGS=" --use-system-libraries"
  rake compile
  rake gem
  git config --global user.name "Foo Bar"
  rake test

Several tests fail (probably due to hardcoding on rugged's side):
ruby-rugged-0.28.4.1-libgit-0.99.0.log

When using libgit2 0.28.4 sources for the tests it currently fails on several integration tests, that seem signature/certificate related and I assume, that something was changed in a remote repository, that is used for testing (which breaks reproducibility):
ruby-rugged-0.28.4.1-libgit-0.28.4.log

Even more bizarre is the fact, that the tests that used to work are now segfaulting, when building rugged 0.28.4.1 against the library and sources of libgit2 0.28.4 (still our current version):
ruby-rugged-0.28.4.1-libgit-0.28.4-current.log

Reproduce the issue with the documented rake compile, rake gem, and rake test commands using libgit2 0.28.4 and 0.99.0 with system libraries enabled. Start by comparing the three attached test logs, then trace the failing integration tests and segfault; done means the relevant test suite runs without these failures or crashes for the affected library versions.