bash: Complete long options to git-add.

bash: Complete long options to git-add.

The new --interactive mode of git-add can be very useful, so users
will probably want to have completion for it.

Likewise the new git-add--interactive executable is actually a
plumbing command.  Its invoked by `git add --interactive` and is
not intended to be invoked directly by the user.  Therefore we
should hide it from the list of available Git commands.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>