ENH: Record a dataset's ID in a superdataset on add/save (fixes gh-3303)

[mih]

Otherwise we have incomplete identity information on all dataset components (i.e. we only have the state of a subdataset, but not its persistent ID). This is an issue during metadata extraction, where we would require subdatasets to be around in order to be able to give a full report.

We already ensure that create(force=True) does not alter a dataset's ID, so getting out of sync would require manual fiddling with DataLad's internal config. Sure be safe...

[yarikoptic]

Should be ok... but the code which starts relying on it (probably should later add check to install/clone) should assure to not crash if there is no ID recorded.

[kyleam]

Makes sense.

The --add in the git config call made me pause, but I suppose it doesn't matter in practice.

[mih]

Well, tests are going nuts in a few places. Apparently datalad/interface/tests/test_ls_webui.py tests on scenarios where .gitmodules is an annexed file. Not yet sure whether this makes any sense or why it is happening.

[mih]

I think I should move this into rev_save(). It should still catch all relevant cases, but not run counter to more simple-minded command assumptions

[codecov]

Codecov Report

Merging #3304 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3304      +/-   ##
==========================================
+ Coverage   91.03%   91.04%   +<.01%     
==========================================
  Files         263      263              
  Lines       34221    34229       +8     
==========================================
+ Hits        31154    31163       +9     
+ Misses       3067     3066       -1

Impacted Files	Coverage Δ
datalad/core/local/tests/test_create.py	100% <100%> (ø)	⬆️
datalad/support/gitrepo.py	89.1% <100%> (+0.06%)	⬆️
datalad/interface/run_procedure.py	89.87% <0%> (+0.63%)	⬆️

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[mih]

Finally!