Fix two ifs

Only two lines changed.

Fix an if for GitRuby::Internal::LooseStorage    
    If sha1 is e.g. 'fie5f' then sha1[2..39] is "e5f".
    Even if sha1 is 'fi' the sha1[2..39] is (oddly) ""
    and not nil. So the line
        return nil unless sha1[0...2] && sha1[2..39]
    gets passed with any string longer than 1 char
    which apparently shouldn't be the case.
    (tried this with ruby 1.8.7 and 1.9.2)

Fix an if for GitRuby::Repository
    If sha1o is not String it probably doesn't have the method
    'empty?' and the if
      if sha1o.nil? || sha1o.empty? || !sha1o.is_a?(String)
    breaks right there. Also, if sha1o is nil 'is_a(String)' covers
    that.