scons: Rewrite git style hook installer

scons: Rewrite git style hook installer

The SCons script currently assumes that .git is a proper directory
with all git meta data. This isn't the case if the working directory
was checked out using git worktrees. In such case .git is a file with
meta data telling git where the repository data is stored.

This changeset updates changes the SConstruct file to rely on git
rev-parse to get the real git directory.

Change-Id: I3d0475eabc12e868193797067a88e540a9b6e927
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>