eeschema: ERC flags as an error when a hierarchical pin is connected to a bus (lp:#1831594)

Original report created by Dino Ghilardi (dino-ghilardi)

Passing a bus from a child sheet to its parent using hierarchical sheet
label is recognized as an error by the eeschema DRC:

ErrType(19): Invalid connection between bus and net items
   @ (217,17 mm, 36,83 mm): Horizontal Bus from (217,17 mm, 36,83 mm) to
(246,38 mm, 36,83 mm) and Hierarchical Sheet Pin BUS_1_out are graphically
connected but cannot electrically connect because one is a bus and the
other is a net.

***** Sheet /Sheet5CEC1DA2/


Application: Eeschema
Version: (5.1.0-846-ga5f670b69), release build
Libraries:
    wxWidgets 3.0.2
    libcurl/7.52.1 OpenSSL/1.0.2r zlib/1.2.8 libidn2/0.16 libpsl/0.17.0
(+libidn2/0.16) libssh2/1.7.0 nghttp2/1.18.1 librtmp/2.3
Platform: Linux 4.9.0-8-amd64 x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.0.2 (wchar_t,wx containers,compatible with 2.8) GTK+ 2.24
    Boost: 1.62.0
    OpenCASCADE Community Edition: 6.8.0
    Curl: 7.52.1
    Compiler: GCC 6.3.0 with C++ ABI 1010

Build settings:
    USE_WX_GRAPHICS_CONTEXT=OFF
    USE_WX_OVERLAY=OFF
    KICAD_SCRIPTING=ON
    KICAD_SCRIPTING_MODULES=ON
    KICAD_SCRIPTING_PYTHON3=OFF
    KICAD_SCRIPTING_WXPYTHON=ON
    KICAD_SCRIPTING_WXPYTHON_PHOENIX=OFF
    KICAD_SCRIPTING_ACTION_MENU=ON
    BUILD_GITHUB_PLUGIN=ON
    KICAD_USE_OCE=ON
    KICAD_USE_OCC=OFF
    KICAD_SPICE=ON

Original tags: eeschema erc