blame: Print the filename with blame #last

blame: Print the filename with blame #last

In principle we could have got this from "git blame" but (1) that
makes parsing this more annoying and (2) by rights this should come
from the fish cfg so that it can be a display name at some point.

The main effect is to print something like:
  13:41 <Servus> Modified flirts 2011-05-31: new fish from Alice
rather than:
  13:41 <Servus> Modified 2011-05-31: new fish from Alice

I have not taken the trouble to touch the three open-coded special
cases for blame #trouts etc.  I thought that was beyond the call of
duty.  In those cases it will print:
  13:41 <Servus> Modified fish? 2011-05-31: new fish from Alice

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>