git-svn: Don't create a "master" branch every time rebase is run

git-svn: Don't create a "master" branch every time rebase is run

If you run "git-svn rebase" while sitting on a topic branch, there is
no need to create a "master" branch if one didn't exist already. The
branch was created implicitly by the automatic checkout after fetching,
which in the case of rebase isn't actually necessary anyway.

Signed-off-by: Steven Grimm <koreth@midwinter.com>
Acked-by: Eric Wong <normalperson@yhbt.net>