Handle attachment replacements

[mkoeppe]

Currently we only keep the latest version of an attached file.

[14:40:50] INFO     Migrating ticket #34113 ( 19 changes): "Drawing Schlegel diagrams of 4-polytopes using tik"                                                                                                migrate.py:1973
           INFO     Attachment filename='hypercube.png' mimetype='image/png'                                                                                                                                   migrate.py:1646
           INFO     Replaced by filename='hypercube.png.gz' mimetype='application/gzip'                                                                                                                        migrate.py:1661
           INFO     Attachment link https://github.com/sagemath/sage/files/ticket34113/hypercube.png.gz                                                                                                        migrate.py:1684
           INFO     Attachment filename='hypercube.png' mimetype='image/png'                                                                                                                                   migrate.py:1646
           INFO     Replaced by filename='hypercube.png.gz' mimetype='application/gzip'                                                                                                                        migrate.py:1661
           INFO     Attachment link https://github.com/sagemath/sage/files/ticket34113/hypercube.png.gz                                                                                                        migrate.py:1684
           INFO     Attachment filename='hypercube.png' mimetype='image/png'                                                                                                                                   migrate.py:1646
           INFO     Replaced by filename='hypercube.png.gz' mimetype='application/gzip'                                                                                                                        migrate.py:1661
           INFO     Attachment link https://github.com/sagemath/sage/files/ticket34113/hypercube.png.gz                                                                                                        migrate.py:1684

[mkoeppe]

Actually these are just multiple references to the same attachment; the new warning is never issued.