git-svn: Correctly handle root commits in mergeinfo ranges

git-svn: Correctly handle root commits in mergeinfo ranges

If the bottom of a mergeinfo range is a commit that maps to a git root
commit, then it doesn't have a parent.  In such a case, use git commit
range "$top_commit" rather than "$bottom_commit^..$top_commit".

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