support/scripts/pkg-stats: extract current commit id, not master

support/scripts/pkg-stats: extract current commit id, not master

pkg-stats extracts the Buildroot commit id from which the package
information was collected. However, when doing so, it always assumes
we're using the master branch, by running "git log master".

But in fact, pkg-stats can be run from any branch/tag, so it makes a
lot more sense to use "git log HEAD".

Cc: victor.huesca@bootlin.com
Cc: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>