>From within a git worktree, "git describe" failed due to the explicit
--git-dir option:

      GEN      stamp-revision
    fatal: not a git repository: '../../.git/'

Let git find the actual git dir by just providing the path (-C) from
which we want to execute the git command.

Signed-off-by: Steve Lhomme <robux4@ycbcr.xyz>