When using a push.default = current central workflow [1], we should use
@{push} instead to locate the remote branch.
In fact, @{push} covers most cases in push.default = upstream too. The
branch.<name>.merge is probably only needed when using RemoteURL and
different remote / local branch names.
[1] https://github.com/tpope/vim-fugitive/issues/1172#issuecomment-522301607
