"break" in interactive rebase confuses edamagit

[bmartin]

If you use "b" for break in an interactive rebase, magit status will forget that a rebase is happening. b at the start of the file is a really convenient way to start before your first commit. For example, if I want to spell-check a file before my change.

I also see similar behavior when using "x" for "exec".

Reproduction:

1. Start an interactive rebase. For example, "r -i u"
2. Add a "b" for "break" on a line by itself between two lines in the rebase
3. Start rebase
4. Observe that when the break happens, magit status no longer shows anything about the ongoing rebase
5. run git rebase --continue in the terminal

Note this does not happen with edit.