scm-wrapper: Allow running command after git clone
    
    When a file should be obtained from a git repository, allow running an
    arbitrary command (like `make`) after clone but before copying the files
    out. This only works for the Git backend.
    
    The downside is that a clone is needed and we can no longer use `git
    archive` to speed things up.
    
    Fixes: https://pagure.io/pungi/issue/5
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>