fix reflog parsing

[Byron]

When the reflog message contains >, the previous greedy parsing of the actors signature would cause too much to be consumed.
The idea is to always parse until \t which naturally separates the message from the well-defined parts.

@epage maybe you could have a look at this, as I am pretty sure that I have butchered winnow and I don't feel good about it.
The difficulty for me is to 'limit' what the actor parser can see at the \t location, while leaving the actor parsing as is. Maybe it's possible to 'pre-parse' from the right to '\t' so that
everything else naturally doesn't go beyond \t.

Thanks for your help.

[Byron]

I am going to merge this to include the fix in the upcoming release. However, I hope that we will get to update the code to use winnow facilities eventually, right now it's a hack to workaround my ignorance.