remote: handle rename of remote without fetch refspec

remote: handle rename of remote without fetch refspec

We return an error when trying to rename a remote that has no fetch
refspec:

  $ git config --unset-all remote.origin.fetch
  $ git remote rename origin foo
  fatal: could not unset 'remote.foo.fetch'

To make things even more confusing, we actually _do_ complete the config
modification, via git_config_rename_section(). After that we try to
rewrite the fetch refspec (to say refs/remotes/foo instead of origin).
But our call to git_config_set_multivar() to remove the existing entries
fails, since there aren't any, and it calls die().

We could fix this by using the "gently" form of the config call, and
checking the error code. But there is an even simpler fix: if we know
that there are no refspecs to rewrite, then we can skip that part
entirely.

Reported-by: John A. Leuenhagen <john@zlima12.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>