Fix "tg base -h" and generalize "tg base".

All TopGit must output a usage message when supplied the "-h" option,
or "tg help subcmd" will not work.  Fix "tg base".

Allow "tg base" to receive a number of TopGit-controlled branches, and
print the base for each of them.

Signed-off-by: Per Cederqvist <ceder@lysator.liu.se>
Signed-off-by: Robin Green <greenrd@greenrd.org>