Implement equals() and hashCode() on PermissionBackendCondition

We want to be able to deduplicate PermissionBackendConditions when given
a list of such objects. Therefore, this commit implements equals() and
hashCode().

This is not trivial since the individual implementations of
PermissionBackendCondition encapsulate a CurrentUser as well as a
PermissionBackend. Both of which do not implement equals() and
hashCode().

This is correct, since PermissionBackend is a service and comparing it
to other PermissionBackends makes no sense. CurrentUser could be
compared to other CurrentUser objects but the outcome depends on
wheather two annonymous users should be considered equal or not. This
depends on the use case.

Therefore, this commit adds user() and resourcePath() to
PermissionBackend to get the entities that a PermissionBackend object is
scoped to and evaluate the case at hand directly in
PermissionBackendCondition.

resourcePath() can come in handy in other places as well since it is an
accurate representation of the resource that we are performing checks
on. Concrete PermissionBackend implementations can use it to communicate
with their remove service if desired.

The implementation for resourcePath() picks /+ as the delimiter since it
is a forbidden character combination for project names. An alternative
would be to URL-encode the project name, but this is more expensive.

Change-Id: I7b357e61bfc6f14acc7b0d06830b615847798ec3

java/com/google/gerrit/server/permissions/DefaultPermissionBackend.java

@@ -70,6 +70,11 @@ public class DefaultPermissionBackend extends PermissionBackend {
       this.user = checkNotNull(user, "user");
     }
 
+    @Override
+    public CurrentUser user() {
+      return user;
+    }
+
     @Override
     public ForProject project(Project.NameKey project) {
       try {