git-gui: Display status on left in diff header.

git-gui: Display status on left in diff header.

Because the Tk pack layout manager gives all space to the right/bottom
most widget during expand/contract of the frame we were adding and
removing all space from the status area of the bar and not from the
file name, which is what we actually wanted.

A simple enough fix is to just put the status of the given file on
the left side of the diff viewer header rather than on the right.

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