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.