devel/libgit2: Add python as a build dep to fix NINJA_DEFAULT=samurai

Normally, python is brought in from ninja. When NINJA_DEFAULT=samurai,
that dependency is missing.

• Could NOT find PythonInterp (missing: PYTHON_EXECUTABLE)

CMake Error at tests/libgit2/CMakeLists.txt:7 (message):

Could not find a python interpreter, which is needed to build the tests.
Make sure python is available, or pass -DBUILD_TESTS=OFF to skip building
the tests

Note that python is only used to build tests, so no PORTREVISION bump is
needed here.