Add --no-rebase option to git-svn dcommit

git-svn dcommit exports commits to Subversion, then imports them back
to git again, and last but not least rebases or resets HEAD to the
last of the new commits. I guess this rebasing is convenient when
using just git, but when the commits to be exported are managed by
StGIT, it's really annoying. So add an option to disable this
behavior. And document it, too!

Signed-off-by: Karl Hasselström <kha@treskal.com>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>