git-svn: Demonstrate a bug with root commits in mergeinfo ranges

git-svn: Demonstrate a bug with root commits in mergeinfo ranges

If a svn:mergeinfo range starts at a commit that was converted as a
git root commit (e.g., r1 or a branch that was created out of thin
air), then there is an error when git-svn tries to run

    git rev-list "$bottom_commit^..$top_commit"

because $bottom_commit (the git commit corresponding to r1) has no
parent.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Acked-by: Eric Wong <normalperson@yhbt.net>