Differential tarball tool

[ilanschnell]

This PR adds the conda-diff-tar command to conda-mirror, a tool for creating tarballs from a previous reference point of a conda repository. The file diff-tar.md explains all the details.

[codecov]

Codecov Report

Merging #9 into master will increase coverage by 1.57%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master       #9      +/-   ##
==========================================
+ Coverage   88.78%   90.36%   +1.57%     
==========================================
  Files           2        3       +1     
  Lines         312      415     +103     
==========================================
+ Hits          277      375      +98     
- Misses         35       40       +5     

Impacted Files	Coverage Δ
conda_mirror/diff_tar.py	95.14% <0.00%> (ø)

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[ilanschnell]

Ouch, I should probably add some tests.

[scopatz]

Shouldn't this be a subcommand of conda-mirror, rather than a separate command?

[ilanschnell]

Not sure, I realized that just adding the new functionality as options wouldn't make too much sense. Having a sub command it of course possible too, for example conda-mirror diff-tar.

[scopatz]

@xhochy @mariusvniekerk - any opinions?

[ilanschnell]

Thanks for the feedback @scopatz. I've addressed the points. The only remaining question is how the new functionality should be invoked. First, I though that adding options to the existing conda-mirror command would be best, but then I realized that the new functionality is really for a very special (and not very common) case and adding the options to the existing command would probably only be confusing, in particular since most users are never going to need the functionality. This is why I added the conda-diff-tar command. Another option would be to add a sub-command conda-mirror diff-tar, but since conda-mirror doesn't have any sub-commands so far, this would be awkward too. Thoughts?

[ilanschnell]

Thanks @scopatz for suggesting to raise an error and handling it in main. I've made that change, and also added a test for the error being raised in read_reference() when the reference file is not present. I hope we are ready to merge this now.

[xhochy]

Thanks @ilanschnell !

[scopatz]

I am going to cut a release with this capability