redhat/scripts/rh-dist-git.sh does not take any arguments: fix error message

if [ -z "$RHDISTGIT_BRANCH" ]; then echo "$0 [local clone] [alternate tmp] [alternate dist-git server]" >&2; exit 1;

The above is wrong since

commit e2abc56bcaf0ed772e9f2c828342a5ab8fcd6f1f
Author: Prarit Bhargava <prarit@redhat.com>
redhat/scripts/rh-dist-git.sh: Use Makefile variables

Replacing it with

echo "$0: RHDISTGIT_BRANCH is not set" >&2

Signed-off-by: Denys Vlasenko dvlasenk@redhat.com

Fixes: #100 (closed)