Rename 'backout' to 'upstream'

The name is much more meaningful.

Signed-off-by: Gustavo L. de M. Chaves <gnustavo@cpan.org>
Create aliases for 'checkout' and 'upstream'

Now, 'checkout' can also be called 'co' and 'upstream' can also be
called 'up'.

Signed-off-by: Gustavo L. de M. Chaves <gnustavo@cpan.org>
Implement option --delete for 'upstream'

This option forces the deletion of a change-branch after the checkout
of its upstream, even when it's a new change. However, the --keep
option has precedence over it.

Signed-off-by: Gustavo L. de M. Chaves <gnustavo@cpan.org>