At a quick glance, EventBus and FlaggedRevs are the two extensions using
the hashes. EventBust just puts them into the emitted data; FlaggedRevs
detects reverts to the latest stable revision that way (so there is no
rev_sha1 based lookup in either case, although in the case of FlaggedRevs I
could imagine a use case for something like that).
Files on the other hand use hash lookups a lot, and AIUI they are planned
to become MCR slots eventually.
For a quick win, you could just reduce the hash size. We have around a
billion revisions, and probably won't ever have more than a trillion;
square that for birthday effect and add a couple extra zeros just to be
sure, and it still fits comfortably into 80 bits. If hashes only need to be
unique within the same page then maybe 30-40.