git-svn.perl - bug in lookup_svn_merge()

I triggered a bug in lookup_svn_merge when I was trying to convert one of our repositories.

Namely the $bottom variable has sometimes value '1', i.e. the first commit ever. Then it pushes "$bottom_commit^..$top_commit" to some list for further processing. This processing fails, because there is no commit before the first one.

We should test existence of parent of $bottom_commit, but I don't know what is the correct behavior if it does not exist.