gitk should maybe keep focus on same file when switching patch->tree view

if you're looking at the patch for a given file (have it selected in
the lower right window), then when
you switch to tree it would be nice if it started at that file.  I
realize you can view Old version/New version but that doesn't help
with landing the the right level of the tree for projects with
subdirs, and sometimes you want to just switch to looking at the file
sans all diff markup.

Britton