git-gui: Make the copyright notice serve double duty.

The copyright notice we display in the about dialog should be the same
as the one at the top of our source code.  By putting the copyright
notice that appears at the top of our source code into a global variable
rather than a comment we can trivially make them the same at all times.

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