ENH add --data option to get command, matching push one

[bpinsard]

Partially addresses #7468 by adding a --data option to get command with values similar to push command.
Passes --want-get here to git-annex get if auto or auto-if-wanted (and preferred content is set) values are passed.

PR checklist

• Write tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.29%. Comparing base (864dc4a) to head (a6b7271).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7476      +/-   ##
==========================================
- Coverage   91.35%   91.29%   -0.07%     
==========================================
  Files         325      324       -1     
  Lines       43442    43211     -231     
  Branches     5778     5783       +5     
==========================================
- Hits        39685    39448     -237     
- Misses       3741     3748       +7     
+ Partials       16       15       -1     

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[yarikoptic]

I feel that introduction of data potentially makes get_data obsolete and thus require replacement/deprecation cycle and that could be taken as a good thing since get_data flag semantic is already shown to be "inconsistent" (get_data in python but --no-data in CLI).

But -n (AKA --no-data) is used heavily in handbook etc, and is a convenient shortcut to signal --data=nothing.

So, I think that is how -n|--no-data should be promoted - that it is a shortcut for data="nothing" and there should be explicit statement that data option is ignored if --no-data is provided.

But also let's wait for others to chime in

[adswa]

I have placed this PR on the agenda of the next dev call for October 24th, hopefully we can revive the discussion. :)

[codeclimate]

Code Climate has analyzed commit a6b7271 and detected 0 issues on this pull request.

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[bpinsard]

Hi! Any chance this PR can move forward.
We have a usecase that would highly benefit from that feature!

Thanks!

(It seems the mac and lint tests are broken in the master branch, not cause by these changes).