Check out manifest under .repo_manifest

This is part of a workaround for Kokoro where it does not properly clone
the manifest to .repo.

Change-Id: Icf09905342226b061952245cff2324da596eed2a
diff --git a/default.xml b/default.xml
index 0375db3..f84daa6 100644
--- a/default.xml
+++ b/default.xml
@@ -30,4 +30,9 @@
   <project path="third_party/linux" name="hafnium/third_party/linux" clone-depth="1" />
 
   <project path="project/reference" name="hafnium/project/reference" />
+
+  <!-- Checkout this manifest as one of the projects. This will be used to init
+       `repo` in Kokoro builds. Shallow clones are not supported by `repo`, so
+       clone with a large depth. -->
+  <project path=".repo_manifest" name="manifest" clone-depth="1000" />
 </manifest>