tg-checkout: push previous parent child terms aside to make next pop

tg-checkout: push previous parent child terms aside to make next pop

Clear up the continuing confusion around next, prev, push, pop, parent
and child.  Prefer the "next" and "prev" terms as their behavior
matches the TopGit "next" and "prev" commands quite closely.

Include copious documentation updates to make it painfully obvious
what the terms mean and paint some pretty pictures too.

This change deals with the verbiage more so than making any
substantial behavioral changes -- those are left for a future
change.  However, the new aliases "n", "p" and "previous" are
now accepted, but that is the extent of the behavioral change.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>