author information vanishes with multiple log buffers

I often want to have multiple log buffers open at the same time (e.g. in order to compare history for different branches/repositories). This used to work in the past by first creating a log buffer (e.g. "l l"), renaming it to something else and then opening another one.

In today's "next" branch this still works. The bug, however, is that the buffer opened first loses its author information (the right-side column) as soon as the next log buffer is created. It seems that there can only be a single author column across all of Emacs' open magit buffers. There's even an error message `(wrong-type-argument window-live-p nil)` which, according to the backtrace, has something to do with the log buffer not being in an expected state.

All of this makes having multiple log buffers open at the same time pretty much impossible (a shame!).

How to reproduce:

1. Start magit on a repository.
2. Open a log buffer, e.g. `l l`.
3. Rename the buffer with `M-x rename-buffer`.
4. Go back to magit and try to open another log buffer with `l l`.