socorro-syms: Correct the extraction of repository branch

When a recipe has git-cloned source code to compile, it may be checked out with
a detached HEAD. The current VCS information extraction in socorro-syms
incorrectly assumes a detached HEAD belongs to the master branch.

With the proper git command we find the branch the detached HEAD belongs to and
use that in the Socorro link.

Signed-off-by: Daniel Markus <daniel.markus@leica-geosystems.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>