git-gui: Select subcommands like git does.

git-gui: Select subcommands like git does.

If we are invoked as `git-foo`, then we should run the `foo` subcommand,
as the user has made some sort of link from `git-foo` to our actual
program code.  So we should honor their request.

If we are invoked as `git-gui foo`, the user has not made a link (or
did, but is not using it right now) so we should execute the `foo`
subcommand.

We now can start the single commit UI mode via `git-citool` and also
through `git gui citool`.

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