test both git apply and patch

[flavorjones]

Currently, the test suite picks git apply if git is installed, else patch.

On both Travis and Appveyor, as well as any likely developer machine, this means that git is tested but patch is not.

I bring this up because gnuwin32's patch.exe on Windows does not handle embedded whitespace in a patch's candidate file name, and so TestCook#test_patch and TestCookWithBrokenGitDir#test_patch both fail on Windows when git is not installed.