Standardize REST endpoints for getting changes

The /detail and /review endpoints are practically identical to the
standard change endpoint except for the default set of options. Make
this equivalence explicit in the code by delegating to a GetChange
instance from the other handlers.

This change naturally gets caching for the other endpoints as well.

Allow specifying options on /change/X; prior to this, users could add
options to the default set from /detail but could not remove options.

Change-Id: I03bb96141e79ae00359f8d5ec51413d1b27204dc

gerrit-server/src/main/java/com/google/gerrit/server/change/RevisionResource.java

@@ -15,6 +15,7 @@
 package com.google.gerrit.server.change;
 
 import com.google.gerrit.extensions.restapi.RestResource;
+import com.google.gerrit.extensions.restapi.RestResource.HasETag;
 import com.google.gerrit.extensions.restapi.RestView;
 import com.google.gerrit.reviewdb.client.Account;
 import com.google.gerrit.reviewdb.client.Change;
@@ -23,7 +24,7 @@ import com.google.gerrit.server.IdentifiedUser;
 import com.google.gerrit.server.project.ChangeControl;
 import com.google.inject.TypeLiteral;
 
-public class RevisionResource implements RestResource {
+public class RevisionResource implements RestResource, HasETag {
   public static final TypeLiteral<RestView<RevisionResource>> REVISION_KIND =
       new TypeLiteral<RestView<RevisionResource>>() {};
 
@@ -56,6 +57,14 @@ public class RevisionResource implements RestResource {
     return ps;
   }
 
+  @Override
+  public String getETag() {
+    // Conservative estimate: refresh the revision if its parent change has
+    // changed, so we don't have to check whether a given modification affected
+    // this revision specifically.
+    return change.getETag();
+  }
+
   Account.Id getAccountId() {
     return getUser().getAccountId();
   }