bumpspec: don't write changelog if nothing changed.
    
    This should make bumpspec more "idempotent".  Previously, if you
    ran this command twice:
    
        $ rpmdev-bumpspec -n 1.2.3 myfile.spec
        $ rpmdev-bumpspec -n 1.2.3 myfile.spec
    
    You would get two changelog entries for version 1.2.3-1.
    
    With the change in this commit, the first run will add a changelog
    entry, but the second run will do nothing.