scripts: Make git intercept global

The previous minimially invasive git intercept simply isn't enough. For example,
meson used in the igt-gpu-tools recipe hardcodes the path to git in the configure
step so at install time, changing PATH has no effect.

There are lots of interesting things we could do to try and avoid problems but
making the git intercept and dropping fakeroot privs for git global is probably
the least worst solution at this point. It will add slight overhead to git calls
but we don't make many so the overall impact is likely minimal.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>