git-synchronizer: disallow blowing away all the branches

In case a server died (such as pacific.mpi-cbg.de) and the IP is changed
for a name and a new repository is set up, that new repository should
not mean that all the branches should be deleted (just because there was
a previous record of branches and now the remote repository is empty).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>