git-sizer can fail on some very long lines

On some repositories, we can get the following error:

```
% git-sizer --verbose
error: error scanning repository: copy-oids: bufio.Scanner: token too long
```

It appears that one of the lines from `git rev-list --objects` is too long to be read. In the (private) repository instance we saw, the line was 12583034 bytes in size. It is speculated that this is due to a very long filename or path.

git-sizer should handle this more gracefully, if only by printing the affected object ID. Perhaps an option could also be added to adjust the buffer size so that normally git-sizer would use fewer resources but it could be adjusted to deal with larger lines if need be.