Support bash completion on symmetric difference operator.

Support bash completion on symmetric difference operator.

Now that log, whatchanged, rev-list, etc. support the symmetric
difference operator '...' we should provide bash completion for it
just like we do for '..'.

While we are at it we can remove two sed invocations during the
interactive prompt and replace them with internal bash operations.

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