Fix test of existing tags with shallow fetch

Fix test of existing tags with shallow fetch

In case of shallow fetch, there is no local information about all
existing tags. Therefore the test if the tag already exists, should use
git ls-remote in this case. It enables to fail earlier before the try to
push existing tag.

Unfortunately git-ls-remote returns with 0 even if the refs is not
found. So it is necessary to check if its output is not empty.