R1:c5fa7421c222

Allow commits to be queried by repository using the `tagged(...)` typehaead function

Summary:
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…