Inline Edit: Acquire content and type in one request

BinaryResult can return X-FYI-Content-Type header with the
real content type. Use this inside of FileContentUtil to
send back the actual type in one request.

By using a single request the server only needs to load
object data into memory once, to determine type and to
stream the content to the client.

Move base64 decoding into RestApi. The format is very
predictable from the server and can be applied to any
REST API call made from the browser.

Change-Id: I3b6c83efd0eb76148a3ac3e50cd2e3e4c37f08c5

gerrit-server/src/main/java/com/google/gerrit/server/project/BranchResource.java

@@ -41,4 +41,8 @@ public class BranchResource extends ProjectResource {
   public String getRef() {
     return branchInfo.ref;
   }
+
+  public String getRevision() {
+    return branchInfo.revision;
+  }
 }