git-gui: Prefer Tk's entry widget over a 1 line text field.

git-gui: Prefer Tk's entry widget over a 1 line text field.

I'm a fool and previously used a text widget configured with a height
of 1 and special bindings to handle focus traversal rather than the
already built (and properly behaved) entry widget.

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