clone --branch: refuse to clone if upstream repo is empty

Since 920b691 (clone: refuse to clone if --branch
points to bogus ref) we refuse to clone with option
"-b" if the specified branch does not exist in the
(non-empty) upstream. If the upstream repository is empty,
the branch doesn't exist, either. So refuse the clone too.

Reported-by: Robert Mitwicki <robert.mitwicki@opensoftware.pl>
Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>
Acked-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>