Documentation: git-svn: fix example for centralized SVN clone

The example that tells users how to centralize the effort of the initial
git svn clone operation doesn't work properly. It uses rebase but that
only works if HEAD exists. This adds one extra command to create a
somewhat sensible HEAD that should work in all cases.
Signed-off-by: NJan Krüger <jk@jk.gs>
Acked-by: NEric Wong <normalperson@yhbt.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-svn.txt

@@ -522,6 +522,8 @@ have each person clone that repository with 'git-clone':
 	git remote add origin server:/pub/project
 	git config --add remote.origin.fetch '+refs/remotes/*:refs/remotes/*'
 	git fetch
+# Create a local branch from one of the branches just fetched
+	git checkout -b master FETCH_HEAD
 # Initialize git-svn locally (be sure to use the same URL and -T/-b/-t options as were used on server)
 	git svn init http://svn.example.com/project
 # Pull the latest changes from Subversion