I don't believe that shelvesets themselves have any information regarding branches or server paths in them. I've assumed here, that by "shelvesets pertaining to a specified branch/folder" you mean shelvesets that contain files located under the server path for that folder, in that branch. If that's the case, then may be forced to iterate over the files in all your change and test each of their server paths for containment in your branch/folder of interest.
Getting shelveset items from the command prompt is a bit of a chore, as it is unsupported either through tf.exe, tfpt.exe, or the powershell cmdlets; however, I've just resolved a script to do just this in the following thread:
Get Tfs Shelveset file contents at the command prompt?
Give it a try--hope it helps!