MergeUploads could short-circuit if given only one upload

It's possible to end up with MergeUploads being given only a single upload. For example, in the first case I outlined in !1229 (comment 539851), it might not be obvious that if you set upload_merge_uploads: True then a worker will have to install devscripts and run mergechanges just in order to do a no-op merge of a single source upload; the same is also true if you're doing binary-only uploads for a single architecture.

In general, it's awkward for the workflow to avoid this, since it lays out the work request graph before it knows what the results of each lookup are going to be.

I think the MergeUploads task could reasonably notice that it only has a single input upload, and just copy it to the output rather than bothering to install devscripts and run mergechanges.