[MERGE][0.19] Fix passing tracefilename to bzrlib.open_tracefile

[Andrew Bennetts]

Hi James,

I forgot to CC you on the original post.  Are you ok with me landing this for
0.90?

-Andrew.

Andrew Bennetts wrote:
> Currently, if you pass the 'tracefilename' argument to bzrlib.open_tracefile,
> you get an exception.  This simple bundle fixes that.  It was working in 0.17,
> I haven't checked 0.18.
> 
> This is a useful feature if you want to get logging when using bzrlib outside of
> the 'bzr' executable, and don't want to direct it to ~/.bzr.log.
> 
> Unfortunately, with the current structure of bzrlib/trace.py, it is very
> difficult to test this code path, so I have only tested this manually.
> 
> I think this is simple enough that it should go into 0.19, especially as it is a
> regression.
> 
> -Andrew.