git-gui: More consistently display the application name.

git-gui: More consistently display the application name.

I started to find it confusing that git-gui would refer to itself
as git-citool when it was started through the citool hardlink, or
with the citool subcommand.  What was especially confusing was the
options dialog and the about dialog, as both seemed to imply they
were somehow different from the git-gui versions.  In actuality
there is no difference at all.

Now we just call our options menu item 'Options...' (skipping the
application name) and our About dialog now always shows git-gui
within the short description (above the copyleft notice) and in
the version field.

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