Resolve "SHA parameter in accept merge request API"
Add a `sha` parameter to the MR merge API, which must match the source SHA for the branch to be merged.
Also add the same parameter to the UI:
![MR_SHA](/uploads/616da728695dc19fa7ef7ef6a016ff81/MR_SHA.gif)
@DouweM and I discussed adding some smart feature to that, like updating the source SHA on navigating to the diff tab, but for now it will just require a refresh 😃Closes#14139.
See merge request !4414