Redefine TargetTagsDict.get() to always use __missing__
    
    The code uses `target_tags.get(branch, 'rawhide')` and dict.get()
    doesn't use __missing__, only dict[key] access does.
    
    This fixes up 8642f188adbb31a02021334e2016772d12c8e093.
    
    Signed-off-by: Everybody Likes This <everybody@likes.this>