One option is to cover the most recent material in a very superficial level, with very explicit instructions in the text of the question rather than problem solving, so they don't need to be proficient, being clear in advance that that's what will happen.
You could even go to the extent of giving them a sample question saying it's the question on the midterm with some numbers or a function changed, but this still risks focussing their learning too sharply on the new content, which is bad news in the long term.
If you can possibly spare the time, the review class is beneficial since it allows the students to focus on mastery across the content instead of being distracted by trying to figure out the new stuff at whatever level.
They may be prepared to work faster with you afterwards in compensation if you make it clear that you sacrificed the time to help them out with the midterm - you can explain it suits both them and you, since they want good credit and you both want good proficiency.
(One option is to negotiate with the class - the deal is they have to do an extra preparatory piece of work between the midterms and the next class rather than have the weekend or whatever off, to cover material you've sacrificed to help them out. The problem with this is you may have to negotiate this quite some time before they're thinking seriously about revising, since you can't edit the midterm at short notice!)