Embed JSX filename paths relative to repo root (#6778)
Embed JSX filename paths relative to repo root (#6778)

Test Plan: Changed the preprocessor to log the output of babel.transform and saw

```
var _jsxFileName = 'src/isomorphic/modern/element/__tests__/ReactJSXElementValidator-test.js';
```

in the resulting output, instead of an absolute path.
(cherry picked from commit 6afd51061a251f39f7951063f41419917f626f02)

with-git/react,with-git/react,with-git/react,with-git/react,with-git/react,with-git/react,with-git/react