Tweak resolve abbr Git id to return Success/Failure rather than Option
Fix #22 - removal of identically-sized big blobs

Embarrassingly, if you were trying to remove big blobs, and there was more
than one blob of a given byte size, then only *one* of those sized blobs
would be targeted for removal. Caused by incorrect use of a SortedSet,
acting on a comparison operator that reported equality on size equality...

Problem spotted thanks to the magnificient gentleman programmer
@PetrK-GNM, who had a repo that contained two zip files of exactly
61464170 bytes. Petr, we salute you with this earth-shatteringly
inadequate ascii-art rendering of your portrait:

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 :KK0OOOkkOO000KKKKXXXXXXK0OO000OOOkkO00kOOKXXXXKo;....',cloddox0x,
 :k00dxkkOO00000KKKXXXXXXXXXKK000OOO0000000KXXXXKOkxddlccc;cx000Kk'
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  000kxdxkOO0000KKKKXXXXXXXXXXXXXXXKK000K000KKXXXKKKK000000KKKKKKo
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    :kxdxxxxxkO00K00KKKXK00OOOOOOO0OOkddxkOOOOO00KXXXXXXXXXXKKd
    .x00XXXKKKKKKK00KKK0Oxoolllddxdodcocldoxxdk0KKKKKXXXXXKKK0.
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        .c;'.'':xOOOOOO0000000Okkdc;''cddxk00KKK00000Odl
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          .:l:::;:clcodkxOkxdxdxxkxdkxxxkO0KK00OOkxc:'
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                                      ..''.':cc.
                                       ...':;.
                                       .''.
Tweak switch name to 'massive-non-file-objects-sized-up-to' for issue #20
sbt plugin update-fest