Fix scripts/hooks/post-checkout

Shell scripts must use LFs.  Also make it executable.

(Both failures probably don't happen on some windows environment where
git is told to translate EOLs, and all files are assumed executable.)

Author: elibarzilay

scripts/hooks/post-checkout

@@ -1,2 +1,2 @@
-#!/bin/sh
-npm run gulp -- generate-diagnostics
+#!/bin/sh
+npm run gulp -- generate-diagnostics