build-perf-test-wrapper.sh: store buildstats under git-notes
In order to dramatically reduce the data footprint of the result
branches. This makes cloning/pulling result repositories a lot faster.
Buildstats can still be accessed, when needed by doing
git fetch refs/notes/buildstats/*:refs/notes/buildstats/*
[YOCTO #10582]
Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
