a `merge.abort` command which deletes the branch and pops you back; (same as `branch.delete <merge branch>`)

[aryairani]

Even though we can currently use branch.delete as a workaround, it requires potentially a lot of typing which may or may not be easily tab-completable, depending on other existing branch names and merges in progress. A separate command would still be good. (undo for such a command would also be good)