testutils/site.py: Support parsing more exotic git versions

We use output of git --version to determine if we can run some tests that rely on features from newer git versions. Usually, we expect the output to be like:

git version 2.17.2

On some platforms, like MacOS, there could be a suffix after the version string, so that it looks something like:

git version 2.17.2 (Apple Git-113)

This causes things to fail like so:

ValueError: invalid literal for int() with base 10: '2 (Apple Git-113)\n'

Fix logic around HAVE_OLD_GIT such that we split the output of git --version without limit on how many times we split. Previously we used to split only twice so the suffixes like (Apple Git-113) are not part of the parsed version.