tools: copying .gitattributes to all subrepositories (fixes windows build)

Fixes #9281

Change-Id: I42ef7105b2a5a99452d804db31eea57a8d6d9d01
Reviewed-on: https://go-review.googlesource.com/2071
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>