Fixes file change watcher to work with git submodules

Summary:When using git submodules the path to the submodule parent is returned rather than the submodule itself and this broke the watcher in my project

For example: if we have `/Users/brent/monorepo` and `/Users/brent/monorepo/product-a`, when we run `jest --onlyChanged` it should search `/Users/brent/monorepo/product-a` but instead it would search `/Users/brent/monorepo/` because `/Users/brent/monorepo/.git/modules/product-a` is returned from `git rev-parse --git-dir` and `path.dirname` for that returns `/Users/brent/monorepo`

You can probably come up with a cleaner solution using some node or git apis that I'm not aware of, just throwing this out there as one possible fix.
Closes jestjs#832

Differential Revision: D3138457

fb-gh-sync-id: 136124a22acc1a921925c09c62d3f83648a27273
fbshipit-source-id: 136124a22acc1a921925c09c62d3f83648a27273

Author: brentvatne

src/lib/git.js

@@ -40,10 +40,14 @@ function findChangedFiles(cwd) {
 function isGitRepository(cwd) {
   return new Promise(resolve => {
     let stdout = '';
-    const child = childProcess.spawn('git', ['rev-parse', '--git-dir'], {cwd});
+    const child = childProcess.spawn(
+      'git',
+      ['rev-parse', '--show-toplevel'],
+      {cwd}
+    );
     child.stdout.on('data', data => stdout += data);
     child.on('close',
-      code =>  resolve(code === 0 ? path.dirname(stdout.trim()) : null)
+      code => resolve(code === 0 ? stdout.trim() : null)
     );
   });
 }