Use local branch name as release when there is no remote
    
    Previously, release name was derived from the remote Git branch
    associated with the active branch. If the remote branch was not set,
    user had to manually specify the --release argument.
    
    To better support cases where a remote repository does not exist,
    default to local branch name in case the remote has not been defined.
    This is useful e.g. if a local Git repository and commits are used when
    creating a new, complicated package.
    
    In case the local branch name does not match any supported release,
    the problem is detected in method load_rpmdefines using the check that
    already checks if the remote branch name matches any release.
    
    Merges: https://pagure.io/rpkg/pull-request/645
    Signed-off-by: Otto Liljalaakso <otto.liljalaakso@iki.fi>