src/git.cc: Fix API compatibility with libgit2 0.28

src/git.cc

@@ -6,7 +6,11 @@ bool Git::initialized = false;
 std::mutex Git::mutex;
 
 std::string Git::Error::message() noexcept {
+#if LIBGIT2_VER_MAJOR == 0 && LIBGIT2_VER_MINOR >= 28
+  const git_error *last_error = git_error_last();
+#else
   const git_error *last_error = giterr_last();
+#endif
   if(last_error == nullptr)
     return std::string();
   else
@@ -244,7 +248,11 @@ boost::filesystem::path Git::Repository::get_root_path(const boost::filesystem::
       throw std::runtime_error(error.message());
   }
   auto root_path = Git::path(root.ptr, root.size);
+#if LIBGIT2_VER_MAJOR == 0 && LIBGIT2_VER_MINOR >= 28
+  git_buf_dispose(&root);
+#else
   git_buf_free(&root);
+#endif
   return root_path;
 }