git-gui: Reverted file name text field to a label.

git-gui: Reverted file name text field to a label.

So although a text field with a flat relief looks like a label on
Windows it doesn't on Mac OS X.  The Aqua version of Tk is still
drawing a border around the text field and that makes the diff pane
header look pretty ugly.

Earlier I had made the file name area into a text widget so the user
could highlight parts of it and copy them onto the clipboard; but with
the context menu being present this isn't quite as necessary as the user
can copy the file name to the clipboard using that instead.  So although
this is a small loss in functionality for non-Mac OS X systems I think it
is still reasonable.

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