gnu: libgit2, python-pygit2: Update to 0.25.1 [security fixes].

See <http://www.openwall.com/lists/oss-security/2017/01/10/5>.

* gnu/packages/version-control.scm (libgit2): Update to 0.25.1.
* gnu/packages/python.scm (python-pygit2): Update to 0.25.0.