Fixes T12322. Allows you to search for commits using the tagged(...) repository function, so you can find "any commmit in any repository tagged with android" or similar.
I moved the function from Differential (which was the application using it) to Diffusion (which is more accurately the application which provides it).
I fixed a bug where searching for tagged(xyz) would have no effect (constraint was ignored) if there were no repositories tagged with "xyz". The fix isn't perfectly clean, but should work properly for the moment.