OPT+RF: create-sibling --since=^ -- diff to HEAD, use "^" instead of ""

[yarikoptic]

Two commits:

OPT: create_sibling - diff to HEAD not None (tree)

Primary motivation is very slow --since= mode of operation of create-sibling
on a heavy superdataset (see #6399).  Although it could be argued that there might be
a benefit from create-sibling to be able to create siblings for not yet committed
submodules, I think such use cases are sparse if any and thus diff"ing to the committed
state (HEAD) is actually the desired effect.

Fixes #6399 (although I did not check explicitly)

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RF: create-sibling --since to take "^" instead of ""

to be consistent with "push" and thus reduce cognitive load.  Old value of an
empty string will remain supported.  While at it, added a tiny bit of testing
to ensure that --since actually works out the way we expect and only deals with
that subdataset (or notneeded)

Moreover, apparently --since= was not even documented before, this fixes it.

First commit I would not even mind to destine against maint but decided not to bother since 0.16.0 is imminent, right?

Changelog

💫 Enhancements and new features

• create-sibling --since=^ mode will now be as fast as push --since=^ to figure out for which subdatasets to create siblings.

🪓 Deprecations and removals

• create-sibling will require now "^" instead of an empty string for since option.

📝 Documentation

• --since=^ mode of operation of create-sibling is documented now

[codeclimate]

Code Climate has analyzed commit fe60cd4 and detected 0 issues on this pull request.

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[codecov]

Codecov Report

Merging #6436 (fe60cd4) into master (b868dbc) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6436      +/-   ##
==========================================
+ Coverage   89.87%   89.95%   +0.08%     
==========================================
  Files         348      348              
  Lines       43764    43772       +8     
==========================================
+ Hits        39331    39377      +46     
+ Misses       4433     4395      -38     

Impacted Files	Coverage Δ
datalad/distribution/create_sibling.py	65.14% <100.00%> (+0.40%)	⬆️
datalad/distribution/tests/test_create_sibling.py	77.57% <100.00%> (+0.15%)	⬆️
datalad/customremotes/tests/test_archives.py	89.28% <0.00%> (-0.65%)	⬇️
datalad/support/tests/test_annexrepo.py	97.97% <0.00%> (+0.13%)	⬆️
datalad/distributed/tests/test_ria_basics.py	97.91% <0.00%> (+0.29%)	⬆️
datalad/distributed/ora_remote.py	31.57% <0.00%> (+0.32%)	⬆️
datalad/local/add_archive_content.py	88.13% <0.00%> (+0.42%)	⬆️
datalad/local/rerun.py	96.41% <0.00%> (+0.65%)	⬆️
datalad/runner/tests/test_witless_runner.py	95.39% <0.00%> (+0.65%)	⬆️
datalad/downloaders/base.py	82.47% <0.00%> (+0.68%)	⬆️
... and 6 more

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[yarikoptic]

It is magically green! Even nearly all benchmarks under 1 - can't be bad ;-)

[mih]

Let's not let this unicorn event slip!

[yarikoptic]

note FTR: damn, burnt myself -- thought that it was against maint, spent time tracking down in "maint". uff.