BF: Implement workaround for file:// URL compatibility on Windows

[mih]

The new 'compatibility' argument of get_local_file_url() is relatively
painless and the workaround implementation can be removed (and the
argument rendered into a no-op) if there should be a compatibility
fix at the git-annex end.

Fixes gh-3639

[mih]

Ha, so even if we generate proper Git-compatible file:// URLs on Windows this doesn't work, because GitRepo.clone() throws them away and takes a wrong localpath. Here is the relevant code:

       # try to get a local path from `url`:
        try:
            url = url_ri.localpath
            url_ri = RI(url)
        except ValueError:
            pass

It is unclear to me why a file:// URL cannot be passed as-is (but see below for a clue). In any case, with this code gone, things start working.

Tracing the evolution of a URL through the code is a bit funny. There is also this step in clone.py:

        # Possibly do conversion from source into a git-friendly url
        # luckily GitRepo will undo any fancy file:/// url to make use of Git's
        # optimization for local clones....
        source_url = source
        source_ = _get_git_url_from_source(source)

which converts from str to RI to str, which is later converted back into RI.

I will disable this optimization step on windows to get basic functionality with file:// URLs.

[codecov]

Codecov Report

Merging #3979 into master will decrease coverage by 0.18%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3979      +/-   ##
==========================================
- Coverage   89.67%   89.49%   -0.19%     
==========================================
  Files         271      271              
  Lines       36351    36339      -12     
==========================================
- Hits        32599    32521      -78     
- Misses       3752     3818      +66

Impacted Files	Coverage Δ
datalad/distribution/clone.py	90.41% <ø> (ø)	⬆️
datalad/tests/utils_testrepos.py	92.92% <100%> (-0.89%)	⬇️
datalad/support/gitrepo.py	89.15% <100%> (-0.31%)	⬇️
datalad/support/network.py	86.63% <100%> (ø)	⬆️
datalad/support/tests/test_gitrepo.py	99.78% <100%> (ø)	⬆️
datalad/support/tests/test_network.py	100% <100%> (ø)	⬆️
datalad/customremotes/base.py	69.25% <0%> (-14.89%)	⬇️
datalad/customremotes/tests/__init__.py	91.66% <0%> (-8.34%)	⬇️
datalad/support/keyring_.py	84.44% <0%> (-6.67%)	⬇️
datalad/tests/test_base.py	96.55% <0%> (-3.45%)	⬇️
... and 9 more

---

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[kyleam]

The new 'compatibility' argument of get_local_file_url() is relatively
painless and the workaround implementation can be removed (and the
argument rendered into a no-op) if there should be a compatibility
fix at the git-annex end.

Yes, painless, as well as non-intrusive for non-Windows platforms, so it sounds fine to me.

[mih]

Thx @kyleam