Rest API contribution get doesn't return display_name
You're in a bit of a tough spot, I'm afraid.

The display name is attached to the contact, not the contribution.
You would typically use an API join to handle this.  However, joining to the Contact entity isn't supported in APIv3.  It is supported in APIv4 (and in fact is one of the main reasons behind API4).
However, API4 doesn't have a REST interface at present.

So you have two options:

Make 2 REST calls;
Try to use APIv3 chaining.  This will let you make a single REST call, but does NOT reduce the number of API calls made internally.  I've always found it to be more trouble than it's worth though, and I shudder to think about what that would look like over REST.