The maintainers of libgit2 reverted a breaking constness issue in
the git_commit() functions in version 1.8. This issue was reverted for
1.9; so, we'll only apply the fix for 1.8 specifically.

In libgit2 1.9, the structure of the git_config_entry type changed to
remove the public free() property so that it is no longer a part of
the public interface. Instead, a new abstraction was added called
git_config_backend_entry that allows the custom backend to set the
free function.

We updated the config backend implementation to support both the old
and new structures.

We cannot implement reflogs currently due to lack of support in the
system API for libgit2. Recent versions of the library removed one
of the system headers we were using. This patch allows the library to
compile while still keeping the reflog erroring out as unimplemented.

In recent versions of libgit2, a config backend having level
GIT_CONFIG_HIGHEST_LEVEL is marked as readonly. To allow the tests to
pass, we use a different level option.