Don't error out on upload if we are only replacing a patch set

The "repo upload" client always sends us a refs/for/$branch command,
even if the client is only trying to add a new patch set to an
already existing change entity.  It does so because this simplifies
the client logic from needing to handle the case of "replacing
everything" vs. "replacing some, creating some".

We shouldn't consider it an error if the client didn't ask us to make
anything new, but did supply at least one patch set replace to us.

Noticed-by: jbq
Signed-off-by: Shawn O. Pearce <sop@google.com>

appjar/src/main/java/com/google/gerrit/server/ssh/Receive.java

@@ -456,7 +456,7 @@ class Receive extends AbstractGitCommand {
       log.error("Invalid pack upload; one or more objects weren't sent", e);
     }
 
-    if (toCreate.isEmpty()) {
+    if (toCreate.isEmpty() && addByChange.isEmpty()) {
       reject(newChange, "no new changes");
       return;
     }