git-gui: Use borders on text fields in branch dialog.

git-gui: Use borders on text fields in branch dialog.

On Mac OS X wish does not draw borders around text fields, making the
field look like its not even there until the user focuses into it. I
don't know the Mac OS X UI standards very well, but that just seems
wrong.  Other applications (e.g. Terminal.app) show their input boxes
with a sunken relief, so we should do the same.

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