tg help: <something>: improve readability
Previously tg help was not showing Usage line, and with this change, now it
looks like e.g.:
$ tg help import
Usage: tg [...] import [-p PREFIX] RANGE...
Import commits within the given revision range into TopGit,
creating one topic branch per commit, the dependencies forming
a linear sequence starting on your current branch.
The branch names are auto-guessed from the commit messages
and prefixed by t/ by default; use '-p PREFIX' to specify
an alternative prefix (even an empty one).
Signed-off-by: Kirill Smelkov <kirr@landau.phys.spbu.ru>