Handle closing source named branch from a merge request
ATM, when sending a MR from a named branch, Heptapod doesn't really handle the case where you want to close the source branch after merging:
- The UI doesn't offer an option to close the source branch
- When I close the branch from the command line before merging, it automatically closes the MR (see pypy/pypy!702 (closed))
Note that the merge UI has a disabled box labeled "Remove source branch". @marmoute suggested turning it into "close source named branch".