Provide a useful body when revision parsing is ambiguous

Change-Id: I845fef6da5f0e01461f2e4be351a39170196b67b

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

@@ -95,4 +95,13 @@ public class RevisionResource implements RestResource, HasETag {
   Optional<ChangeEdit> getEdit() {
     return edit;
   }
+
+  @Override
+  public String toString() {
+    String s = ps.getId().toString();
+    if (edit.isPresent()) {
+      s = "edit:" + s;
+    }
+    return s;
+  }
 }

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

@@ -15,6 +15,7 @@
 package com.google.gerrit.server.change;
 
 import com.google.common.base.Function;
+import com.google.common.base.Joiner;
 import com.google.common.base.Optional;
 import com.google.common.collect.FluentIterable;
 import com.google.common.collect.Lists;
@@ -84,10 +85,16 @@ public class Revisions implements ChildCollection<ChangeResource, RevisionResour
         match.add(rsrc);
       }
     }
-    if (match.size() != 1) {
+    switch (match.size()) {
-      throw new ResourceNotFoundException(id);
+      case 0:
+        throw new ResourceNotFoundException(id);
+      case 1:
+        return match.get(0);
+      default:
+        throw new ResourceNotFoundException(
+            "Multiple patch sets for \"" + id.get() + "\": "
+            + Joiner.on("; ").join(match));
     }
-    return match.get(0);
   }
 
   private boolean visible(ChangeResource change, PatchSet ps)