git-svn: -h(elp) message formatting fixes

'graft-branches' is slightly longer than the rest of the
commands, so the text was squished together in the formatted
output.  This patch just adds some more whitespace to make
the text look more pleasant.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>