git-svn: add `fetch` subcommand

pages/common/git-svn.md

@@ -6,6 +6,10 @@
 
 `git svn clone {{http://example.com/my_subversion_repo}} {{local_dir}}`
 
+- Fetch new revisions from remote SVN repository:
+
+`git svn fetch`
+
 - Update local clone from the upstream SVN repository:
 
 `git svn rebase`