git-gui: Pad new branch name input box.

git-gui: Pad new branch name input box.

The new branch name input box was showing up too close to the labelframe
border, it was basically right on top of it on Windows.  This didn't
look right when compared to the Starting Revision's expression input
field, as that had a 5 pixel padding.

So I've put the new name input box into its own frame and padded that
frame by 5 pixels, making the UI more consistent.

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