git-gui: Maintain the same file list for diff during refresh.

git-gui: Maintain the same file list for diff during refresh.

I just noticed that a file was always jumping to compare against HEAD
and the index during a refresh, even if the diff viewer was comparing
the index against the working directory prior to the refresh.  The
bug turned out to be caused by a foreach loop going through all file
list names searching for the path.  Since $ui_index was the first one
searched and the file was contained in that file list the loop broke
out, leaving $w set to $ui_index when it had been set by the caller
to $ui_workdir.  Silly bug caused by using a parameter as a loop
index.

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