BinhostService: RegenBuildCache sans git operations.

Update the RegenBuildCache service to be able to operate without
performing the git operations itself. Instead, return a list
of overlays that have modifications that need to be committed.

BUG=chromium:986281
TEST=run_tests, manually ran endpoint

Change-Id: I5737b3f7a8928f976e1294310a20b3facf12cb06
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1754680
Tested-by: Alex Klein <saklein@chromium.org>
Commit-Queue: Alex Klein <saklein@chromium.org>
Reviewed-by: Evan Hernandez <evanhernandez@chromium.org>
diff --git a/api/controller/binhost_unittest.py b/api/controller/binhost_unittest.py
index 12fa0c2..017e7fc 100644
--- a/api/controller/binhost_unittest.py
+++ b/api/controller/binhost_unittest.py
@@ -7,6 +7,8 @@
 
 from __future__ import print_function
 
+import mock
+
 from chromite.api import api_config
 from chromite.api.controller import binhost
 from chromite.api.gen.chromite.api import binhost_pb2
@@ -121,7 +123,7 @@
     input_proto.overlay_type = binhost_pb2.OVERLAYTYPE_BOTH
 
     binhost.RegenBuildCache(input_proto, self.response, self.api_config)
-    regen_cache.assert_called_once_with('both')
+    regen_cache.assert_called_once_with(mock.ANY, 'both')
 
   def testRequiresOverlayType(self):
     """RegenBuildCache dies if overlay_type not specified."""