git-gui: Use a grid layout for branch dialog.

git-gui: Use a grid layout for branch dialog.

Using a stack of frames in the Starting Revision section of the new
branch dialog turned out to be a mess.  The varying lengths of each
label caused the optionMenu widgets to be spread around the screen
at unaligned locations, making the interface very kludgy looking.

Now we layout the major sections of the branch dialog using grid
rather than pack, allowing these widgets to line up vertically in
a nice neat column.  All extra space is given to column 1, which is
where we have located the text fields.

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