BF: obtain information about annex special remotes also from annex journal

[yarikoptic]

An alternative to #6133 implementation
which first forces git-annex to commit.

The use case was detected while working on datalad/datalad-crawler#112
to accompany #6105 .

Although we could have easily "fixed" the test in datalad-crawler, I consider
the approach of going around git-annex commands to interrogate git-annex
metadata to be fragile, in particular for such a case as here: some other
command could have been ran in alwayscommit=false mode, and information in
git-annex branch might not yet reflect actual state of git-annex. Ideally we
should avoid doing such "going around git-annex", but ATM there is no git-annex
command which would "expose" all this information.

[codecov]

Codecov Report

Merging #6135 (ad7c39c) into maint (dccedaf) will decrease coverage by 29.78%.
The diff coverage is 65.41%.

❗ Current head ad7c39c differs from pull request most recent head 9a9a542. Consider uploading reports for the commit 9a9a542 to get more accurate results

@@             Coverage Diff             @@
##            maint    #6135       +/-   ##
===========================================
- Coverage   85.97%   56.19%   -29.79%     
===========================================
  Files         312      312               
  Lines       42215    42494      +279     
===========================================
- Hits        36296    23878    -12418     
- Misses       5919    18616    +12697     

Impacted Files	Coverage Δ
datalad/cmdline/common_args.py	100.00% <ø> (ø)
datalad/cmdline/helpers.py	41.52% <0.00%> (-29.57%)	⬇️
datalad/core/distributed/tests/test_clone.py	0.00% <0.00%> (-97.05%)	⬇️
datalad/core/distributed/tests/test_push.py	0.00% <ø> (-98.09%)	⬇️
datalad/core/local/tests/test_diff.py	0.00% <0.00%> (-97.24%)	⬇️
datalad/core/local/tests/test_run.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/core/local/tests/test_status.py	0.00% <0.00%> (-97.33%)	⬇️
datalad/customremotes/archives.py	0.00% <0.00%> (-62.15%)	⬇️
datalad/customremotes/tests/test_base.py	0.00% <0.00%> (-93.69%)	⬇️
datalad/distributed/export_to_figshare.py	26.41% <0.00%> (+0.16%)	⬆️
... and 261 more

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[mih]

This LGTM. Thx!

However, I want to remind about a comment from Joey

Also reading .git/annex/journal/ will miss configs of private remotes, which are stored in a separate journal and never reach the git-annex branch.

This should be mentioned in the docs, at least. We are not using this yet, but soon "ephemeral clones" will be private annex repos, and then any special remote configured in them would still be invisible to the method. Ping #5835