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