nixos/version: Try to get Git revison from .git

Let's first try if we can determine the Git revision from the .git
directory and if that fails, fall back to get the info from the
".git-revision" file... and after that use something generic like
"master".

This should address #17218 in better way, because we don't need to
create another redundant file in the source checkout of nixpkgs.

I'm not going to route of falling back to using .git, because after
55d881ee

, we already have ".git-revision" files in people's Git
repositories, which in turn means that nixos-version will report that
old file every time even if the working tree has updated.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @bennofs, Profpatsch
Reported-by: @devhell
Fixes: #17218