Fix commit dialog crash when using libgit2 to commit

After associating config with repository, do not call git_config_free() after calling git_repository_free()

git_repository_free() calls git_config_free() internally

Fix regression of 9673785ccf300f424ac3ddd04cf8508eb93ce696

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>