git-gui: Cleanup blame::new widget initialization

git-gui: Cleanup blame::new widget initialization

A lot of this code was pre-class, which meant that I just sort of
copied and pasted my way through it, rather than being really smart
and using a variable for each widget's path name.  Since we have a
field for each path, we can use those throughout the constructor
and make things a lot neater.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>