ObjectIdMatcher: Custom matcher for sets of ObjectIds

ObjectIdMatcher: Custom matcher for sets of ObjectIds

Parsed requests represent object ids (SHA1) in ObjectId instances but tests
use strings for those ids because they are easier to define.

Create a custom matcher that hides the conversion from string to
ObjectId. Note that this reverses the existing code conversion (it was
transforming ObjectIds into string).

This produces more readable code, consistent with the other hamcrest
assertions.

Change-Id: I47ba1d25557d791fe74fb93c740ff7de9923cc00
Signed-off-by: Ivan Frade <ifrade@google.com>