Don't look recursively for a .git dir

The original GetGitRevisionDescription.cmake script would look for
a .git dir recursively in the parent directories of its invocation;
while a nice feature, it may pick up an unrelated repository if there's
none in the Quassel directory. Because we know exactly where to find
our own .git if it's there at all, we can remove the recursion and
prevent this issue.