git-gui: Added about dialog box.

Created a help menu with an about dialog box.  This about dialog
shows the copyright notice for the application, the fact that it
is covered by the GPL v2.0 or later, the authors, and the current
version of Git it is invoking when users perform actions within it.

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