Unify the source macros
    
    Introduce macros %defcommit and %deftag, which are used to define other
    macros depending on whether Source: is a tag or a commit. The other macros
    (%id and %shortid) can be then used in the same way in both cases.
    This way  the rest of the spec file des not need to know whether we are
    dealing with a tag or a commit.