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>

.gitattributes

@@ -0,0 +1,10 @@
+# Treat all files in this repo as binary, with no git magic updating
+# line endings. Windows users contributing to Go will need to use a
+# modern version of git and editors capable of LF line endings.
+#
+# We'll prevent accidental CRLF line endings from entering the repo
+# via the git-review gofmt checks.
+#
+# See golang.org/issue/9281
+
+* -text