git-svn: detect and fail gracefully when dcommitting to a void

git-svn: detect and fail gracefully when dcommitting to a void

The command

  git svn clone (URL of an empty SVN repo here)

works, creates an empty git repository. I can perform the initial
commit there, but then, "git svn dcommit" says :

Use of uninitialized value in concatenation (.) or string at .../git-svn line 414.
Committing to  ...
Unable to determine upstream SVN information from HEAD history

I guess a correct management of the initial commit in git-svn would be
hard to implement, but at least, the error message can be improved.
First step is something like the patch below, and better would be for
"git svn clone" to warn that it won't be able to do much with the
cloned repo.

Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>