init-repository: pass regexp in raw string to git log --grep

This prevents python from escaping the special characters that
resulted in git not finding the proper shasum for the git-svn-id
regular expression line.

Change-Id: Ic5449db6319b638cf692d2b4427ce5de7cbac66e
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
Reviewed-by: Adam Kallai <kadam@inf.u-szeged.hu>