Functionally identical; shirt_lift is used for any top lifting, unless it's a bikini_lift. The reason it proliferated from ~50 posts to ~450 posts in a year was because of DakuTree's use of the tag for his rapid uploads.
If shirt lift is for any top lifting, then the wiki needs to be updated to reflect that. Also to mention that bikini lift and bra lift are exceptions.
But before doing that, what do we do about sweater lift and vest lift which cover lifting tops that may be independent of a shirt, either because the shirt stays down or because the shirt is missing / hidden under the other object being lifted?
Personally, if we're going to make it generic, I'd flip the alias the other way because there are many tops that don't deserve a special tag that aren't necessarily shirts per se. (Tubetops, haltertops, mail, camisoles, etc.)