root: make TestLicenseHeaders skip files unknown to git

The license header test walked the whole tree, so scratch files,
worktrees, and other untracked local files made "go test ." fail in a
developer checkout. Ask git for the set of tracked files and only
check those, falling back to checking everything (as before) when the
tree is not a git checkout, such as in a release tarball.

Fixes #20740

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I9f3a7c21e5b804d6a2f4c8e19d7b53a6e0c412fd